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Impact of agriculture and urban development onthe community structure of wetland birds in EastCentral MinnesotaChrista R. von BehrenMacalester College, christaruth@comcast.net

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Recommended Citationvon Behren, Christa R., "Impact of agriculture and urban development on the community structure of wetland birds in East CentralMinnesota" (2007). Honors Projects. Paper 5.http://digitalcommons.macalester.edu/biology_honors/5

Impact of agriculture and urban development on the community

structure of wetland birds in East Central Minnesota

Christa von Behren

Advisor: Mark Davis

Macalester College

Spring 2007

2

Abstract

Wetlands are one of the fastest disappearing habitats in America. Many wetlands are also being altered due to the effects of various types of land use. Because wetlands provide important habitat for many types of birds, these species can be affected by changes in wetlands due to land use. The impacts of several wetland features, including wetland size, proximity to other wetlands, and vegetation, on bird communities have been debated in the literature. While some studies have found landscape-level features, such as connectivity to other sites to be the most important factors for explaining bird diversity, others have found within-patch characteristics to be more important. It is also unclear how these variables affect rates of nest predation in wetlands. The purpose of this study was to analyze the effects of several wetland features on wetland bird assemblages and nest predation rates at several spatial scales. Bird surveys, vegetation surveys, and measurements of nest predation were conducted at the Cedar Creek Natural History Area in East Bethel, Minnesota. Landscape analyses were conducted at four different spatial scales. Results showed that wetlands are used extensively, not only by species that breed in wetlands, but by species that breed in other environments as well, particularly by woodland birds. Results also indicated that diversity in vegetation structure is associated with an increase in the number of species using wetlands. Low bird species richness in wetlands was associated with increased amounts of agriculture and urban development, which was due to the reduction in trees in agricultural and developed areas. Unlike studies of upland species, birds responded the same way to urban development as to agriculture in the landscape. Features at both the habitat level and at broader landscape scales were found to be significantly correlated with features of the bird communities, indicating the importance of implementing conservation plans at multiple spatial scales. Results suggest that for restoration and construction of wetlands, increasing the variation in both vertical and horizontal structure within the wetland and in the surrounding landscape will increase the bird diversity within the wetland. The results of this study suggest that further encroachment of development and agriculture on wetlands in East Central Minnesota will lead to a decline in wetland bird diversity, particularly with respect to woodland birds that use the wetlands for foraging purposes. The data suggest that woodland obligates will disappear first from the area, followed by sensitive wetland obligates.

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Table of Contents Page

Foreword…………………………………………………………………………………..5

Part I: A landscape approach to wetlands and wetland birds: a literature review…………………………………………………….…....6

Introduction……………………………………………………………..………....6 Wetland types and functions in the landscape…………….…….………………...8 Wetland types and functions……...…………………...………………......8 Geology and hydrologic cycles…………………………………....…..…..9 Disturbances in wetlands……………………...…………………...……..10 Wetlands in the landscape…………………………………………...…...12 Wetlands at the landscape level: Metapopulations and habitat connectivity………………………...……..14 Wetlands as bird habitat………………………………………………….………17 Habitat selection by birds……………………………………….………..19 Wetland use and selection by birds: Importance of size……………………………………….……………….21 Wetland use and selection by birds: Importance of local and landscape features……………………………...24 Nest predation in wetlands…………………………………………………….…28 Urban development and habitat fragmentation……..……....................................29 Wetland conservation ……………………………………….…………………...33 Summary…………………………………………………………………………34 Part II: Association of local and landscape features with the community structure of wetland birds in East Central Minnesota………………………………………………………………….……………..36

Introduction………………………………………………………………………36 Methods………………………………………………………………..…………40 Study sites………………………………………………………....……..40 Bird surveys……………………………………….……………………..40 Measuring predation…………………………………….……………….41 Habitat analysis…………………..………………....................................42 Landscape analysis...………………………...………………………...…42 Data analysis…………………………….……………………………….43 Results ………………………………………………………………….………..44 Foraging guilds…………………………………………………………..46 Habitat guilds…………………………………….....................................47 Nest predation………………………………………………………..…..48 Combined 2001 and 2006 results………………………………………...49 Water Chemistry…………………………………………………………49

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Discussion…………………………………………………………………….….50 Effects of urban development and agriculture…………………………...50 Habitat guilds………………………………………………….....51 Effects of paved road…………………………………………….54 Foraging guilds…………………………………………………..55 Wetland connectivity………………………………………………..…...56 Effect of wetland size and shape……………..…………...……………...58 Effect of vertical and horizontal diversity...……………………………..58 Effect of wetland cover diversity…...……………………………………59 Nest predation……………………………………………………...…….60 Water chemistry and hydrologic cycles………………………...………..61 Conservation implications………………………………...……………..62 Literature Cited…………………………………………………………………..65 Tables ………….………………………………………………………………...71 Figures……………………………………………………………………………78

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Foreword

For most of my life, I never really thought much about birds. I knew that my

grandmother loved them and they woke me up early every time I went camping with my

family. I didn’t start to appreciate birds until I got to college. All it took was one

ecology class and I was hooked. After taking Ornithology I was ready to put my bird

knowledge to work. I got the opportunity to use my new bird identification skills in the

summer of 2006 as part of an ecology internship with Professor Mark Davis at the Cedar

Creek Natural History Area. Margaret Pettygrove and I decided to conduct a research

project on wetland birds in the area. We had a great time getting wet and muddy, and

even once losing the truck to the mud in an attempt to observe wetland birds. I decided

to continue work on the project during my senior year at Macalester and incorporate data

collected in previous years at CCNHA. This thesis is the product of the work conducted

over the summer and during the school year.

I would like to thank Professor Mark Davis for the opportunity to work at Cedar

Creek this summer and conduct this research. I would also like to thank Mark for all of

the help and advice on this project during the school year. I am grateful to Margaret

Pettygrove for getting up very early with me to survey birds all summer. This project

would not have been possible without her help. I would also like to thank Jerald Dosch

for initially getting me interested in birds and for help during the writing process. I

would also like to thank everyone at Cedar Creek, especially Martha Phillips for

providing water chemistry data.

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Part I: A landscape approach to wetlands and wetland birds

Introduction

Birds have long been used as indicators of environmental change due to their

sensitivity to environmental variables. For hundreds of years canaries have been carried

into coal mines and used to indicate the presence of deadly gases. More recently,

ecologists have monitored the populations of many species of birds to indicate changes in

their habitats. Bird populations have even been monitored to indicate subtle changes in

protected and monitored landscapes (Fitzpatrick, 2004).

Bryce et al. (2002) studied birds in the Willamette Valley in western Oregon to

develop criteria for detecting environmental change and determining specific

environmental conditions from bird presence. Another goal of the study was to evaluate

different bird species for their value as indicators. The greatest species diversity was

found near streams that had experienced the least human impact. Thirty two of the sixty

two bird species studied clearly responded to the disturbance gradient in the plots where

surveys were conducted. In general, a trend of decreased species diversity with increased

disturbance was found. The authors concluded that several species are sufficiently

responsive to change in habitat conditions to be used in the creation of an index for

monitoring environmental change in the Willamette Valley (Bryce et al., 2002).

In a study of the impacts of human disturbance on bird communities in western

Sikkim, India, Chettri et al. (2005) found that there was a significant correlation between

bird species richness and tree diversity in the birds’ habitats. Different bird feeding

guilds also showed different preferences for habitat. A feeding guild is a group of bird

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species that all feed in the same or similar ways. Results suggest that birds in this area

are dependent on the compositional complexity of trees, shrubs, and herbs. Human

disturbances did not have negative effects for all birds. The authors found that the

opening of some areas created a mid-successional stage which was beneficial for some

common species. They conclude that complexity in forest structure in this region is

necessary to maintain suitable habitat conditions for birds in different feeding guilds.

They recommend the incorporation of guild monitoring as an indication of environmental

conditions (Chettri et al., 2005).

In a study conducted in Pennsylvania, O’Connell et al. (2000) also studied the use

of bird guilds in environmental monitoring. Like Bryce et al. (2002), they used bird

guilds to create an index to be used for monitoring environmental change. Birds were

assigned to response guilds. Each response guild was a group of species of birds that

responded to the change in availability of a particular resource. They found that the

response guilds were useful in classifying the amount of disturbance in a habitat.

Response guilds also reflected some physical and chemical conditions present in the

studied habitats. The authors suggested that the developed index based on response

guilds could be used in place monitoring strategies that involve more intense surveying of

bird populations. The index could be used for evaluating environmental conditions at

landscape scales. For more accurate analyses of environmental conditions in landscapes,

they recommended the use of the index in combination with other types of indicators

(O’Connell et al., 2000). Because birds are commonly used to indicate environmental

change, it is important that their habitat requirements and community dynamics are

8

understood. Because many birds use wetlands for breeding and foraging, they have the

potential to be useful indicators of changes in wetland environments.

Wetland types and functions in the landscape

Wetlands are broadly defined as any habitats that are wet for some period of time,

and are often viewed as the transition zones between aquatic and terrestrial environments

(Tiner, 1999). Although scientists often disagree on precisely which features define a

wetland (Hammer, 1997), they are found mainly in areas where surface water collects or

where groundwater discharges (Tiner, 1999). Wetlands vary enormously in their

hydrology, ranging from mainly terrestrial in nature to aquatic (Weller, 1999). As a

result, the determination of a wetland boundary is somewhat arbitrary due to this

enormous variation. As a consequence of hydrological variation, wetlands vary in the

communities of plants and animals that they support (Tiner, 1999).

Wetland types and functions

Hydrologic regime and vegetation characteristics are both used to classify

wetlands. Swamps are dominated by hardwood vegetation, which can be in the form of

shrubs, hardwood trees, or coniferous trees. In contrast, marshes are dominated by

herbaceous vegetation, with little woody vegetation. Swamps and fens typically develop

in shallow basins, while marshes usually have deeper water (Wovcha et al., 1995).

Hydrophytes, plants that live in anaerobic conditions due to standing water or excess soil

water, are often used to determine the presence of a wetland in an area. However,

9

because many of these plants can grow in terrestrial habitats as well, their use as wetland

indicators can be problematic (Tiner, 1991).

In the prairie pothole region of the upper Midwest, many wetlands are

depressional ponds that occur as a result of former glaciations in the region. As the

glaciers receded, blocks of ice were often buried by till. When this ice melted, the

overlying sediment collapsed, leaving holes in the landscape that became wetlands when

filled with water (Wovcha et al., 1995). These wetlands can serve many hydrological

functions, including storage of water received from surface and groundwater, as well as

atmospheric and groundwater recharge. One study (Winter and Woo, 1990) found that

prairie pothole wetlands are not part of the natural system of surface water drainage, and

there is usually no through flow of surface water through stream channels that connects

wetlands. Winter et al. (1984) explained that this lack of surface through flow is why

prairie pothole wetlands can serve water storage functions. The water regime of a

wetland, including the length of time that it holds water, determines the composition of

the plant community in the wetland (Kantrud et al., 1989; Stewart and Kantrud, 1972).

Geology and hydrologic cycles

Because open water in a wetland and vegetation affect the wetland bird

community (Weller, 1999), the geology and subsequent hydrologic conditions that exert

control over the wetlands’ ecology (Hammer, 1997) could affect bird communities. The

underlying geology of a wetland plays a crucial role in the control hydrological function.

The topographic location of the wetland in the landscape affects its interaction with the

local groundwater system. Salinity, which is an important determinant of the wetland

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vegetation composition, is controlled by the geological structure and resulting

hydrological functions. Salinity can also fluctuate throughout the year in a wetland. The

salt concentrations are much higher when the wetland is at its driest in the summer, and

decrease when it receives large amounts of water (LaBaugh et al., 1996). Since salinity

affects the composition of the wetland vegetation, it may indirectly affect the wetland

bird community as well.

Hydrologic regime is a crucial element of a wetland that can exert control over the

system’s ecology. Ultimately, the hydrology of a wetland determines how the system

will function and what organisms can live there (Hammer, 1997). The vegetation present

in a wetland is determined by the flooding regime, drainage characteristics, soil

saturation, soil type, and water table location below the surface. Wetlands are constantly

exchanging water with the atmosphere, surface water, and ground water. The extent to

which each of these water sources interacts with a wetland varies considerably among

locations and to a large extent determines the type of wetland present (Carter, 1999).

Understanding the interactions between the geology, hydrological dynamics, and plant

ecology in a wetland could help to explain the distribution of wetland birds.

Disturbances in wetlands

Disturbances play an important role in many ecosystems because they create

patterns in the vegetation and are natural sources of habitat heterogeneity (Turner et al.,

2001). Disturbance regimes can also affect the suitability of wetland habitats for birds.

Steen et al. (2006) suggest that hydrologic change in wetlands in the Great Lakes region

could have large effects on bird communities. They argue that processes that

11

homogenize the environment, such as water regulation, will reduce the bird population by

reducing heterogeneity, as this feature creates habitat for many species of birds.

Reducing water fluctuations would also likely cause some microhabitats to disappear

entirely. Microhabitats, such as those created by submergent vegetation, that are

specifically associated with wetlands may disappear, and the birds that depend on them

would be left without suitable habitat at that site. Using information from a literature

review, the authors placed wetland bird species into different risk categories according to

their expected responses to water stabilization. Surveys were also conducted, and the

resulting data were used to construct models to determine the accuracy of the assigned

risk categories. The survey data suggest that the proposed risk categories accurately

reflect the response of birds to hydrologic change. Therefore, their results suggest that

hydrological change in the region could be a driver of population trends of these bird

species. The authors hypothesize that anthropogenic regulation of water levels in

wetlands could affect bird populations more than natural mortality factors do because the

birds have not adapted to anthropogenic factors (Steen et al., 2006).

The drawdown of water in managed wetlands was also found to impact diversity

of water birds by Taft et al. (2002). They found that wetlands with periodic water

drawdown supported more bird species than did flooded wetlands. Dabbling and diving

ducks stopped using wetlands at the end of the dewatering cycle. The authors measured

the water depth at which species richness was greatest. In winter, the water bird diversity

was greatest at 10 to 20 cm in depth. At this depth, habitat diversity was also greatest.

These results suggest that the richest bird assemblage coincides with the greatest habitat

diversity. They authors also found that lowering water levels 10 cm below the traditional

12

level significantly increased the number of species and individuals that used a wetland.

They emphasized that the topography underlying the wetland is important because it

influences water depth. Results suggest that the availability of shallow water may be the

limiting factor for several species in winter, and the authors recommend that grassland

managers increase the amount of shallow water available (Taft et al., 2002).

Murkin et al. (1997) also discussed the importance of regular wet-dry cycles in

wetlands for maintaining bird habitat. They monitored several species of birds during

water drawdown and reflooding events. While dry cycles and subsequent reflooding may

temporarily reduce the habitat suitability for some birds the authors emphasize the

importance of these events in maintaining long-term habitat suitability. The presence of

emergent vegetation is dependent on periodic draw downs of water. When water is

permanent and the wetland starts to evolve into more of a lake than a marsh, emergent

vegetation is lost. Because emergent vegetation is crucial for some birds, their habitat is

also lost with the establishment of permanent water. Regular drawdown events will

maintain vegetation diversity and allow for high use of the wetlands by birds. The

authors also emphasize the importance of preserving a variety of wetlands within a

matrix to provide different habitat types for birds (Murkin et al., 1997).

Wetlands in the landscape

Wetlands are naturally patchy, fragmented habitats in the landscape (Hanski,

2005). A landscape is defined by Turner et al. (2005) as an area that is spatially

heterogeneous in at least one factor of interest. A landscape can span meters or

kilometers, and can include both aquatic and terrestrial systems. Landscape ecology

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emphasizes the interactions between spatial pattern and ecological processes. Recently

landscape ecology has emerged as an important field of study that has become integrated

with land management (Turner et al., 2001).

The suitability of a landscape for providing habitat for organisms is species-

dependent. Many different factors determine suitability, and a landscape cannot be

simply classified as suitable or unsuitable. There is a range of habitat suitability, and

different patches within a landscape can vary considerably in their suitability for any

species. Individuals can be found in habitat patches that are less suitable than others and

survive and reproduce in these locations (Turner et al., 2001).

Scale is an important concept in landscape ecology. The term scale refers to the

dimension of an object or process in space or time. There is no single scale that is

appropriate for the study of all ecological problems and processes. Within a landscape,

processes that occur at fine scales can be considered the mechanisms of landscape

dynamics. The broad-scale patterns in the landscape are the constraints which limit the

processes that can occur within that landscape (Turner et al., 2001). Spatial scale is

important to consider in studies of wetland birds because different species have home

ranges at different landscape scales. While some species may only use a portion of a

wetland, others can have home ranges spanning several wetlands in a landscape (Weller,

1999). For wetland communities to be fully understood, they will need to be studied at

multiple spatial scales.

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Wetlands at the landscape level: Metapopulations and habitat connectivity

In fragmented landscapes, ecological studies are often focused on metapopulation

dynamics. As wetlands are naturally fragmented and patchy systems (Hanski, 2005), a

metapopulation approach is often used when studying wetlands. A metapopulation is an

assemblage of many populations of a single species that exist at several sites (Hanski,

2005). If a population is fragmented, containing a network of subpopulations, and each

subpopulation has a small probability of extinction, local extinctions may be balanced by

recolonization from neighboring subpopulations. Thus, local extinctions and

recolonization at the subpopulation level may be dynamic, but regionally stable at the

metapopulation level (Turner et al. 2001). Metapopulations may have important

functions in human-dominated landscapes. Patches in these landscapes are usually not

large enough to maintain isolated populations of species. However, networks of patches

may be able to preserve metapopulations. Verboom et al. (2001) emphasize the need for

indices that can assess whether spatial conditions can allow metapopulations to exist

(Verboom et al., 2001). Unlike relatively stable, large populations, the persistence of a

metapopulation is due to asynchrony in the dynamics of the subpopulations. Species

persist as a result of a balance between extinction and colonization in the

metapopulations. However, some density dependence exists at the subpopulation level,

and is an important factor for determining the long-term persistence of the

metapopulation (Hanksi, 1998).

While metapopulations allow for population persistence in fragmented

landscapes, total habitat area is still important for the success of the population. The

extinction risk of local subpopulations decreases as patch area increases, since larger

15

patches allow for larger subpopulations to persist. For this reason, the author

recommends that conservation efforts focus on preserving the largest amount of habitat

possible (Hanski, 1998).

The connectivity of wetlands in the landscape is an important feature for the

survival of metapopulations of wetland birds. Hanski (2005) defines habitat connectivity

in terms of migration of individuals to different patches. According to Taylor et al.

(1993) and With et al. (1997), landscape connectivity is defined by the functional

relationships among habitat patches due to their spatial distribution and the movement of

species in response to landscape structure (Taylor et al., 1993; With et al., 1997).

Migration among patches is critical for the survival of metapopulations. Different habitat

patches vary in their spatial connectivity. The more connected and easy to reach the

patches within a wetland complex are, the easier it is for birds to find food and other

resources within the complex (Haig et al., 1998).

Connectivity can be a difficult variable to measure because it is the result of the

movement of organisms, not a measurement fixed in space. While the connectivity of

two patches is clearly related to the distance between them, Hanski emphasizes that

distance to a neighboring patch is not a good measurement of connectivity. Near-by

patches will not necessarily have populations that can supply individuals to other patches,

perhaps because the patch is unsuitable for the subpopulation. The patch could also be

suitable, but happens to not be occupied at the time. The near-by patch would not be

supplying individuals to other patches and therefore are not connected through migration

(Hanski, 2005). Turner et al. (2001) suggest that the average distance to other patches

16

can be used as a surrogate measure for connectivity, and is more accurate than simply

measuring the distance to the nearest patch.

Wetlands are naturally fragmented habitats compared to other habitat types. A

single wetland is generally small in size, at least compared to a typical upland habitat,

and separated from other wetlands (Hanski, 2005). However, wetland complexes,

landscapes containing many connected wetland patches, are thought to be important for

many bird populations (Haig et al., 1998). While many bird species migrate large

distances annually, some water birds also move among several wetland patches within a

single breeding season. These movements may be important for offspring survival. It

has been suggested that environmental variability among wetland patches within a

complex is important for these birds. Changes in hydrological regime can create

problems for breeding birds, but variability among patches increases the likelihood that

they will be able to find a patch with favorable conditions. Haig et al. (1998) recommend

a consideration of wetlands not as isolated patches, but rather as connected mosaics for a

better understanding of how they are used by wetland birds (Haig et al., 1998).

Some wetlands can also become seasonally connected hydrologically. Leibowitz

and Vining (2003) studied the temporal connectivity of wetlands through surface water in

the prairie pothole region. They found that several wetlands in their study area became

seasonally connected due to precipitation and local relief characteristics. The authors

suggest that this intermittent connectivity could affects metapopulations in the area.

While most metapopulation studies have focused on movement of individuals over land

or through flight, it is thought that surface water connectivity could facilitate the

movement of other species. This type of movement may be especially important for local

17

seed banks. It is not known how such movement and connectivity affects bird

populations. However, temporal connectivity of surface water could alter habitat

suitability by changing the hydrochemistry of a wetland and the plants found there,

affecting a food source for many bird species (Leibowitz and Vining, 2003).

Wetlands as bird habitat

Wetlands provide habitat for many different kinds of birds and other animals. A

habitat can be defined as simply the home for populations of living organisms (Hanski,

2005). A habitat contains the conditions needed for individuals to survive and reproduce.

In conservation biology, habitat is the most important concept because it is the most basic

requirement for the survival of populations and species (Hanski, 2005).

A bird’s breeding habitat must provide all of the resources that the bird needs to

live and reproduce. These resources include food, water, nest sites, roosting places, and

cover. In every landscape birds have a variety of habitat sites to choose from. Birds are

clearly good at selecting appropriate habitat, as they are rarely found outside of suitable

areas. For some species, the habitat requirements are very specific and only one type of

habitat can be used. Other birds are generalists and can use a variety of habitat types. In

all habitats potentially suitable for birds, the presence or absence of a bird depends on

some limiting factor. There will be some requirement that is rare compared to others,

such as holes in trees for roosting, available prey, or absence of predators. These rare

requirements must be available, and a bird will not choose an otherwise suitable habitat if

it lacks this limiting factor that is needed for survival (Temple, 2004).

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Wetlands provide many important resources for birds, and many birds use them as

breeding habitat. They provide water, which is a necessity for birds for drinking and

bathing. Open water also provides feeding opportunities for some birds, in particular for

diving ducks. Deep water can provide an escape route from predators. Wetlands can

provide resting places and ideal habitat for breeding and molting. In addition, wetland

vegetation can provide locations for escape from predators (Weller, 1999).

Wetlands provide important food resources for many species of birds, including

species that don’t breed in wetlands. A wide variety of food sources, both plant and

animal can be found in wetlands. Much of this diversity in food is a product of the

different conditions produced by hydrological cycles. Both seeds and tubers are

commonly found bird foods in wetlands. Wetland vertebrates, such as fish, frogs, and

small mammals, are also food sources for some wetland birds. The huge number and

diversity in invertebrates is food for many types of birds, especially during the breeding

season (Weller, 1999).

For many birds, wetlands meet the specific requirements for breeding habitat.

Because of the heterogeneity in habitat type in many wetlands, they provide breeding

territory and nesting sites for many species of birds with very different breeding and

nesting behaviors. Ducks use open water for their courtship displays while Red-winged

Blackbird males sing from their territories in the vegetation to attract females. Nests are

placed in all parts of the wetland, from open water to the dry edges of the wetland

(Weller, 1999).

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Habitat selection by birds

Because wetlands are such variable and patchy systems, birds may have several

types to choose from in a landscape. It is not known exactly how birds select their

habitats, but there are clearly both inherited and learned components. It is thought that as

a result of natural selection, birds inherit knowledge of the general range of their habitat

requirements (Temple, 2004). By the very precise selection of habitat, however, it is

clear that some aspects of habitat selection are learned in the early stages of life. A bird

first learns of appropriate habitat characteristics when living in its parents’ territory.

Once it has selected a territory of its own, it learns the precise location of that habitat and

may return to that location year after year (Temple, 2004). While many of the

requirements for suitable habitats are understood, the process of habitat selection by birds

remains one of the central questions in avian ecology (Battin and Lawler, 2006).

Many ecologists have studied patterns of habitat selection by birds. Ambuel and

Temple (1983) studied bird habitat selection in woodlands in Wisconsin. They found that

bird community composition was correlated with patch area (Ambuel and Temple, 1983).

In contrast, Sodhi et al. (1999) found that habitat selection by American Redstarts

(Setophaga ruticilla) in aspen-dominated forest fragments in Alberta was based on

vegetative characteristics. Willow abundance was particularly important in explaining

habitat selection patterns. The birds preferentially selected sites with high willow

abundance and avoided areas dominated by the common and dominant tree and shrub

species. However, shrub cover was found to be an important characteristic of selected

territories, along with the presence of willows. The authors hypothesize that shrub cover

20

is important for reducing predation and parasitism on American Redstart nests (Sodhi et

al., 1999).

Habitat selection patterns have also been studied in wetlands. Weller (1999)

suggests that for wetland birds, the amount of open water in a wetland or wetland

complex is important for habitat selection. Murkin et al. (1997) studied habitat selection

by birds in prairie wetlands over a period of ten years. They looked for cues used by

blackbirds, waterfowl, and American Coots (Fulica americana) when selecting habitats.

The study was conducted in Manitoba in a series of experimental marshes where the

water level was controlled. They found that use of marshes by birds varied according to

hydrological cycles. The seasonal wet-dry cycles affected the availability of cover and

food, among other important resources. There are likely intercorrelations among the

factors driving the selection of particular habitats.

The authors found that the two species of blackbirds, the Yellow-headed

Blackbirds (Xanthocephalus xanthocephalus) and Red-winged Blackbirds responded

differently to changes in marshes, and suggest that this difference is due to competition

between the species. As Picman et al. (1993) also found, the Yellow-headed Blackbirds

preferred the center of the marsh in this study, likely because of reduced nest predation

rates in this area. American Coots preferred habitats with large areas of open water and a

lot of emergent vegetation. In the summer, dabbling ducks also selected habitat with an

abundance of emergent vegetation. Later in the year these ducks tended to forage in

agricultural fields. They continued to use areas with open water and vegetation, likely

because of its value of protection from predators and bad weather. In contrast to the

dabblers, diving ducks used habitats with deep water and little emergent vegetation in the

21

summer. In the fall however, diving ducks became more associated with emergent

vegetation, likely for weather protection.

The wet-dry cycles in the marshes had a large impact on bird use. Use of the

marshes was generally lower during the dry cycles than during other parts of the year.

When the wetland basin is dry, birds nesting there must have access to another water

source. The response by birds to the return of water to the basin is not the same with

each reflooding event. The characteristics of the wetland will differ based on the amount

of water that comes back into the basin. Quick flooding may inhibit the growth of

emergent plants and not attract the birds that rely on this type of vegetation. However

diving ducks that require large amounts of open water will be more attracted to the area

(Murkin et al., 1997). It is clear that the scale of a habitat patch varies for different

species of birds, and that different birds respond to different cues when selecting habitat.

Wetland use and selection by birds: Importance of size

The importance of the size of a wetland for determining the suitability habitat for

birds has been debated by scientists. Naugle et al. (1999) found that some species are

area-dependent. The authors studied the influence of scale in the use prairie wetland

habitats by birds. They hypothesized that different species of birds would show different

perceptions of landscape structure and respond to features at different landscape scales.

They found that for several bird species there was an interaction between wetland area

and internal habitat characteristics which determined suitability of habitat for that species.

The space requirement for a species could not be determined without consideration of the

habitat features within that space. For example, Black Terns (Chlidonias niger) were

22

found to require large areas consisting of several wetlands. However, area requirements

fluctuated in response to the structure of individual wetlands. The distribution of Yellow-

headed Blackbirds and Pied-billed Grebes was (Podilymbus podiceps) explained solely

by patch area and within-patch characteristics, with no influence of surrounding

landscape features. Large wetland area was important for grebes, while the blackbirds

were correlated with the amount of vegetation cover. In contrast, the surrounding upland

characteristics were important in determining Black Tern distribution. The researchers

used the amount of anthropogenic grassland in the upland matrix to estimate pesticide

and fertilizer runoff into the wetlands. Habitat suitability models for the three studied

species showed diverse perceptions of landscape structure, as the authors had predicted.

Wetland size was found to be an important feature that determines habitat suitability for

some species, but other features are important as well (Naugle et al., 1999).

Findlay and Houlahan (1997) also found that the size of a wetland is an important

feature determining habitat value for many species. In their study of southeastern Ontario

wetlands, they found that species richness of birds, mammals, reptiles, amphibians, and

plants increased with wetland area. They conclude that bigger is better from a

biodiversity perspective (Findlay and Houlahan, 1997).

In contrast, Semlitch and Brodie (1998) found that the number of wetlands in a

landscape is more important for birds than the size of an individual wetland. They argue

that small wetlands play large roles in metapopulation dynamics, and that loss of small

wetlands will reduce the biodiversity of both flora and fauna in a landscape. The loss of

any wetland will reduce the number of species in the area. The ability of species to travel

between large wetlands will be hindered by the loss of small wetlands by increasing the

23

distance between wetland sites. Stopover sites where birds can go en route to a larger

wetland will be lost. With reduced migration among wetlands, a diminishing population

in one wetland site will be less likely to be rescued by immigration of individuals from

neighboring populations. The authors recommend that regulations consider not only

wetland size but wetland density in a landscape when determining sites for conservation

(Semlitch and Brodie, 1998).

Paracuellos and Tellería (2004) found that the structural features of a wetland are

important in addition to size. They found that avian richness increased with wetland size

and suggest that this correlation is due to the tendency of habitat heterogeneity to increase

with wetland area (Paracuellos and Tellería, 2004). Riffel et al. (2006) also found

heterogeneity to be important. Forested depressional wetlands were favored by many

birds, both those species characteristic of forests and those characteristic of wetlands.

Golet et al. (2001) found habitat heterogeneity to be an important habitat feature

for birds in maple swamps in Rhode Island. They found that the maple swamps were

dominated by bird species that typically hold territories in the forest interior but also

tolerate edge habitats. Swamp size and edge development were strongly correlated with

bird species richness. Like Paracuellos and Tellería (2006), the authors suggest that

species richness increased with swamp area because habitat heterogeneity increased with

swamp area. The abundance of forest interior specialists was correlated with the amount

of upland forest. These species were negatively correlated with the presence of wetland

within the forest; however edge species were positively correlated with this feature.

There was likely an overall positive effect of the availability of wetland on the total

abundance of birds in the landscape. Swamps likely provide resources for the interior-

24

edge species that upland forests do not. There was also greater bird abundance in areas

with thicker peat deposits. This trend may be due to the correlation of other habitat

features important to birds that are related to peat depth. The authors conclude that there

may be no inherent value to large swamp size. While more birds were found in larger

swamps, no species were found to use exclusively the larger swamps but were present in

small swamps as well (Golet et al., 2001).

Wetland use and selection by birds: Importance of local and landscape features

In a study of Great Lakes coastal regions, specific habitat components were found

to important in explaining the distribution of both wetland and upland species (Riffel et

al., 2006). Upland birds were more likely to use wetlands with large amounts of

graminoid vegetation. Wetland birds were associated with greater water depths and

diverse forest canopies. There was a relationship found between Song Sparrows

(Melospiza melodia) and Downy Woodpeckers (Picoides pubescens) and wetland

features, suggesting that these species benefit from forested wetlands, even though they

are not typically considered to be wetland species. It is clear that the birds benefited from

features not found in their typical habitats. The authors suggest that wetland birds search

for good habitat sites hierarchically, by first selecting large forest tracts, and then

searching for wetlands within them. They recommend the conservation of large forest

tracts with many wetland patches (Riffel et al., 2006).

It is not clear whether landscape or local structural features are more important in

predicting habitat suitability for birds. Naugle et al. (2001) studied the roles of local and

landscape features in predicting habitat suitability for 20 wetland bird species. They

25

conducted bird surveys, vegetation analyses, and landscape analyses. Then, they

simulated the loss of small wetlands in the landscape and studied the resulting effects on

the habitat suitability of the larger wetlands. They found that most of the permanent

wetlands were larger, while most temporal or seasonal wetlands were small. The scale of

importance was found to be species-dependent. For some species, like the Virginia Rail

(Rallus limicola) and Marsh Wren (Cistothorus palustris), habitat suitability depended on

local vegetative characteristics within wetlands.

For other species, such as the Northern Pintail (Anas acuta) and Black Tern,

landscape-level features at broader spatial scales determined the suitability of the habitat.

For these species, it is important to identify the landscape characteristics that determine

habitat suitability, because a wetland with suitable vegetative and hydrological

characteristics will still not be a good habitat for these species if it is surrounded by

unfavorable landscape characteristics. For example, results showed that the Northern

Pintail and Black Tern were more likely to use a wetland surrounded by grassland than

one surrounded by tilled matrix. Preserving a large number of wetlands in a landscape is

important because of the unpredictable nature of wetland hydrology. If one wetland in

the landscape currently has unfavorable hydrologic conditions, birds are still likely to

find suitable habitat in the landscape if there are many wetlands.

Results of the modeling experiments demonstrated the importance of small

wetlands for the suitability of larger wetlands (Naugle et al., 2001). The authors found

that the species most vulnerable to the loss of small wetlands are those that exploit

resources over a broad spatial scale. For Black Terns, feeding occurs over a large area.

When wetlands are small, they become increasingly dependent on wetlands in the

26

surrounding matrix. For this species, preservation of a high density of wetlands in a

landscape is important. For the Northern Pintail, breeding occurs in small wetlands

adjacent to a larger wetland where the birds feed. A large wetland with good food

resources will not be suitable without a nearby smaller wetland. Protected wetlands are

usually large, permanent wetlands, creating a challenge for the preservation of smaller

wetlands. The authors emphasize the importance of preserving landscapes with high

densities of well connected wetlands (Naugle et al., 2001).

Whited et al. (2000) found that wetland connectedness was the most significant

predictor of species richness of birds in depressional wetlands in three ecoregions of

Minnesota. The authors defined connectedness as a contiguous polygon of relatively

unimpacted land cover surrounding the wetland site. This feature is a measure of the

remnant patch size that surrounds each wetland site. Open water was an important aspect

of connectedness in some wetland complexes, but was less important in others. In one of

the ecoregions, high levels of connectedness corresponded with complexes that had

enough landscape diversity to provide requirements for waterfowl during different life

stages. Isolated sites were usually dominated by generalist bird species, while complexes

of well connected sites supported many wetland specialists. Connectedness appeared to

be a good surrogate measure of other wetland variables. Connectedness was also found

to be a better predictor of bird assemblages in agricultural regions that wetland area was

(Whited et al., 2000).

Twedt et al. (2002) found that structural features were important for the

colonization of bottomland hardwood forests by birds. They studied several sites in

Mississippi and Louisiana between two and ten years after replanting. Birds did not

27

colonize stands before the tenth year after trees had been planted. Maximum tree height

and tree cover, which were measurements of vertical structure, were closely linked to the

colonization of the sites by birds. The results indicate that reforestation with fast-

growing, early successional tree species is most beneficial for birds. The authors

recommend replanting in managed areas with a mixture of predominantly fast-growing

species (Twedt et al., 2002).

The relevant scale and features of the landscape, vegetation, and hydrology that

are important for birds depend on the life history traits of each species. Craig and Beal

(1992) studied marsh bird communities in Connecticut. They found that the feature of

importance for a bird species depended on what that species was using the wetland for.

Birds breeding in a wetland corresponded with different characteristics than birds that

foraged in the wetland but did not breed there did. Area was the most important factor

predicting the species richness of wetland breeders. For nonbreeders, the habitat mosaic

was more important than wetland area. There was an association between vegetation and

habitat heterogeneity and nonbreeder species richness. Most of these species used

mudflats for foraging and the marsh surface for resting and feeding. Wetland

connectivity was also important for these species. There was a strong correlation

between nonbreeder species richness and proximity to a neighboring marsh. For birds

searching for ephemeral prey sources, traveling between marshes is likely a more

effective foraging strategy than remaining at one wetland site is (Craig and Beal, 1992).

28

Nest predation in wetlands

Nest predation creates challenges for all breeding birds. One of the benefits of

nesting in marshes may be reduced nest predation rates. Picman et al. (1993) studied

predation on passerine nests in marshes. They compared nest predation rates in marshes

to rates in upland habitats. Overall, predation rates were lower in marshes than in

uplands. However, placement of nests within the marsh had a large effect on predation

rates. Rates were high in the shallow areas of the marsh, lower in areas of medium water

depth, and higher again in deep areas. Marsh Wrens were found to be responsible for

much of the nest predation in deep water areas where they nest. The predation rates on

experimental nests in deep areas were related to the density of Marsh Wrens. Predation

in deep areas was still lower than in shallow areas, however, due to the exclusion of many

upland predators, such as the raccoon (Stewart, 1999). There is a very diverse predator

community in upland areas, while in deep water areas Marsh Wrens are the only nest

predators. The authors also suggest that the lower predation rates in deeper water could

be due to efficient nest defense by Red-winged Blackbirds which nest in this area. While

water depth was important for nest predation rates, distance from the marsh edge was not

found to be important. They conclude that different nest predation patterns in marshes

and in uplands have led to the evolution of different reproductive strategies in these areas.

The high rate of polygyny among marsh birds may be due to safer nesting conditions,

giving males the opportunity to monopolize. They also hypothesize that predation by

Marsh Wrens has led to colonial nesting behavior by blackbirds in deep water areas

(Picman et al., 1993).

29

Nest predation can have a large effect on population dynamics. In their study of

birds in bottomland hardwood forests after reforestation, Twedt et al. (2002) found high

rates of nest predation in some areas. Predation was the primary cause of low nesting

success in these areas, and it was proposed that these areas are population sinks.

Reproductive success does not compensate for mortality in these areas, but there are high

rates of colonization by individuals from other patches (Twedt et al., 2002). Rodewald

and Yahner (2001) also found nest predation to have major effects on avian community

structure. They found greater rates of nest predation near agricultural disturbances

(Rodewald and Yahner, 2001).

Urban development and habitat fragmentation

Understanding the ecological effects of land-use changes remains an important

challenge to ecologists (Turner et al., 2001). Fragmentation of habitat is known to be a

major threat to bird populations in all habitats. Habitat fragmentation involves both loss

of habitat area and loss of connectivity. Habitats become increasingly fragmented as

patch size decreases and as isolation of the habitat patches increases. With increasing

fragmentation, connectivity among patches decreases. With this loss of connectivity,

birds can lose access to habitats. Migration among patches is greatly reduced, which can

eliminate the rescue effect and have consequences for populations (Hanski, 2005).

Sallabanks et al. (2000) studied the effects of fragmentation on bird populations in

bottomland forests in North Carolina. They found that proximity to edges usually had

positive effects on the abundance of species. Certain species appeared to prefer edges.

Surprisingly, results also showed that patch size was not proportional to bird density and

30

species richness. Species characteristic of one habitat were found in another habitat at

the edges, likely because at the edge there is greater proximity to the preferred habitat.

They hypothesize that the effects of fragmentation found in their study were dependent

on the nature of the edges of the fragments. For some species, the effects of patch size

and edge may be uniform across all landscapes, while for others the effects may be

landscape-specific (Sallabanks et al., 2000).

The effects of edges created by fragmentation were also studied by Dowd (1992)

in an assessment of the effects of development on bird species composition in forested

wetlands in Staten Island, New York. The immediate effect of development at the study

site was the fragmentation of a stream corridor by paved roads. Forest habitats are also

removed, creating edges. While the creation of edges results in poor breeding habitat for

many species, it is attractive to species that are more tolerant of disturbances. Two plots

surveyed in this study had the same area but different shapes. The plot that was long and

narrow, with more edge and a greater ratio of edge species to forest interior species. In

addition, more than half of the species in this plot were species attracted by urban

landscapes. In contrast, at the plot with a smaller amount of edge, less than one third of

the bird community was made up of species attracted to urban landscapes. Results of this

study indicated that large habitat patches in urban landscapes can provide nesting areas

for some species, but encroachment of development leads to dominance in the bird

community by species attracted to urban development (Dowd, 1992). DeLuca et al.

(2004) suggested that there is a disturbance threshold for birds. Beyond this amount of

disturbance bird communities will be negatively affected (DeLuca et al., 2004).

31

Road construction can be a cause of habitat fragmentation in many different

landscapes, including in wetland complexes. Findlay and Bourdages (2000) studied the

effects of road construction on biodiversity in wetlands in southeastern Ontario. They

found that for birds, reptiles, amphibians, and plants there were significant negative

effects on biodiversity that appeared only several years after the roads had been

constructed. For birds, they estimated that species loss can be detected within eight years

of construction. The authors suggested that this time lag after occurs because the

population decline in response to road construction occurs slowly. Researchers are not

able to notice a significant change in populations for several years. Because of the time

lag between road construction and the appearance of effects on populations it is important

to determine an appropriate temporal and spatial scale for study when conducting impact

assessments. The authors emphasized the importance of selecting appropriate indicators

for a given type of disturbance. The results of this study suggest that overall species

richness is not a very sensitive indicator of the effects of road construction. The use of

this indicator will likely underestimate the effects of road construction, at least in the

short term. The authors recommend the use of buffer zones between constructed roads

and wetland patches to mitigate the effects of the road (Findlay and Bourdages, 2000).

DeLuca et al. (2004) suggest that roads can create habitat for generalist species, which

can then outcompete wetland specialists.

Development in the surrounding landscape may also affect bird populations in

wetlands. Findlay and Houlahan (1997) found that forest density and paved road density

within two kilometers both affect the species richness of four taxa in wetlands. Both of

these factors had the largest effects 1000 to 2000 meters from the edge of a wetland.

32

With increased road density, bird, plant, reptile, and amphibian species richness

decreased. With increased forest cover, the species richness of reptiles, amphibians and

mammals increased, but bird species richness was not significantly affected (Findlay and

Houlahan, 1997).

Agriculture can also have large effects on wetlands and on wetland birds. The

runoff of chemicals and sediments into wetlands from agricultural fields is one of the

largest causes of wetland degradation (Stewart, 1999). Gleason et al. (2003) found that

sediment loading in wetlands can affect populations of plants and insects in the wetland.

Seeds and plants were buried by sediment, and the emergence of plant seedlings and

invertebrates was halted. Invertebrates were found to be more sensitive to burial by

sediment than were seeds. The sediment may disrupt cues for the invertebrates to emerge

from diapause, in addition to creating a physical barrier to emergence. A large sediment

load in a flooded wetland may also inhibit productivity in the wetland by reducing the

amount of light that penetrates to the aquatic plants for photosynthesis. Results of this

study show that agricultural activities close to wetlands could limit the reestablishment of

plants and invertebrates. More research is needed to determine an acceptable amount of

sediment loading for the emergence of these organisms, and to determine the relationship

between specific agricultural practices and sedimentation (Gleason et al., 2003). It is

unclear how sedimentation from agriculture affects wetland bird communities, but a

significant reduction in plant and invertebrate production in a wetland could affect the

food available for birds.

In wetland complexes, wetlands that are not adjacent to farmland can still be

affected by agricultural inputs. Whigham and Jordan (2003) emphasize that wetlands in

33

the prairie pothole region are rarely hydrologically isolated. They are connected through

groundwater or intermittent surface water, and each wetland therefore has the potential to

affect other systems (Whigham and Jordan, 2003). Nutrient deposition due to

agricultural practices can create special problems in this linked system. Wetlands

enriched with nitrogen and phosphorus have a high potential to discharge the nutrients to

downstream systems (Winter et al., 2001). It is not clear how the addition of nutrients

into wetland systems affect bird communities.

There are very complex relationships that exist between terrestrial and aquatic

systems. Studies of toposequences have shown that ecosystem adjacencies play

important roles in the transfer of nutrients (Turner et al., 2001). Adjacent cities and

agricultural areas can contribute to nonpoint source pollution that enters a wetland or

wetland complex. Nitrogen and phosphorus are two of the most important pollutants that

move from land to watersheds (Turner et al., 2001).

Wetland Conservation

In the past 200 years, half of the wetlands in the United States have disappeared

(Dahl, 1990). The Clean Water Act of 1972 made a major attempt to combat the rapid

loss of wetlands. The benefits of wetlands and the harmful effects of draining them have

only recently been realized. Restoration and creation of wetlands are now being used to

restore these benefits. The goals of wetland restoration and creation are to maintain and

restore certain ecological functions while accommodating human development. Usually

wetlands are created for the purpose of storing and filtering water, especially in and near

urban areas. While wetland restoration and creation can be very beneficial for many

34

different populations, there are many challenges that arise. Restoring one function, such

as water storage, could eliminate another ecological function of the wetland. Restoration

could also destroy habitat for some species, even though species diversity may be

enhanced and more habitat may be created. Site selection for wetland creation can also

be a challenge. The geological setting will determine what kind of wetland can be

created in an area and will determine the hydrological function of the wetland. It is also

important to establish plant communities that will enhance hydrological functions of the

wetland, stabilize the soil, and provide resources for animal communities (Kentula,

1999).

Recently, the Army Corps of Engineers has been charged with measuring the

value of wetlands that are not immediately adjacent to navigable rivers. The corps has

been asked to generate criteria by which to determine whether wetlands provide

significant enough benefits to water quality to prevent their development. Due to the

incredible diversity, the development of one set of criteria seems an unrealistic task, and

many fear that this will lead to a reduction in federal protection of wetlands. Scientists

say that birds and other indicators can help determine whether a wetland is positively

affecting water quality, but now they are being asked to determine whether these benefits

are important enough to merit protection (Stockstad, 2006).

Summary

The dynamics of wetlands and wetland bird communities are complex and

variable. Geological and hydrological characteristics create enormous differences in

wetland vegetation and water features. Different bird species respond to different

35

environmental variables at different spatial scales. It is still unclear how land-use

changes interact with habitat-level variables to affect wetland bird communities. The

dependence of different species on features at different spatial scales creates special

challenges for conservation. More research is needed to determine ways to restore and

construct wetlands to meet the needs of many different types of birds while also serving

hydrological functions. If birds are to be used as indicators of environmental change and

to evaluate the progress of conservation projects, their habitat requirements and responses

to land use patterns need to be better understood. To help provide this information, I

conducted a study near the Minneapolis-Saint Paul Twin Cities metropolitan area in East

Bethel, Minnesota to evaluate the association of local and landscape-level features of

wetlands with bird distribution in wetlands surrounded by a range of landscape types,

including other wetlands, wooded areas, agricultural fields, and urban development.

36

Part II: Impact of agriculture and urban development on the

community structure of wetland birds in East Central

Minnesota.

Introduction

Wetlands of the prairie pothole region provide important breeding and foraging

habitat for many species of birds. However, wetlands are also quickly disappearing

ecosystems in North America. They are second only to grasslands in percentage of total

area lost since European settlement (Weller, 1994). In the past several decades, there

have been efforts to restore altered wetlands and to construct them in locations where

they previously did not exist (Kentula, 1999). Wetlands are also often constructed for

hydrological purposes in, but usually fail to support a diverse bird community (Athanas,

1988). More research on the important wetland habitat characteristics for birds is needed

for wetland restoration and construction efforts to preserve habitat for diverse bird

communities as well as for hydrological function.

Several wetland characteristics have been found to affect the composition of

wetland bird communities. Many studies have shown that landscape characteristics

surrounding the wetlands, in addition to within-site characteristics are important

determinants of the community composition of many wetland birds. Some researchers

have found wetland complexes to provide habitat for many more bird species than

isolated wetlands do (Fairbairn 2001, Haig, et al., 1998, Naugle et al., 2000). However,

the important features and scales of analysis wetland complexes for birds are not

37

understood. Haig et al. (1998) described landscape connectivity as a functional

relationship among habitat patches caused by their spatial distribution and the movement

of organisms in response to landscape structure (Haig et al., 1998). Understanding the

characteristics of functional complexes of wetlands used by birds is important for the

conservation of wetland habitat. It is also necessary to understand the features at the

habitat level that are related to bird community composition.

The relative importance of wetland vegetation, size, and shape for bird

populations is not clear. Riffel et al. (2001) found most studied bird species to be

significantly correlated with increasing wetland area, and recommends emphasis on large

wetlands for bird conservation programs. In contrast, Sallabanks et al. (2000) found

wetland size to be less important in bird abundance than other characteristics.

Paracuellos and Tellería (2004) found wetland size to be an important determinant

of the number of bird species found in a wetland, and the degrees of wetland patchiness

and isolation were important as well. The authors suggest that bird species richness is

greater in large wetlands because habitat heterogeneity usually increases with wetland

area. They recommend focusing conservation efforts on maintaining a large number of

wetlands, paying special attention to large and clumped ponds (Paracuellos and Tellería,

2004). Haig et al. (1998) also found environmental variation to be important for bird

diversity, but in contrast to Paracuellos and Tellería, the authors focused on variation

within a complex of wetlands. Variability within a complex allows birds to easily find

new habitat when the conditions in their current patches change (Haig et al., 1998).

Naugle et al., (2001) found small wetlands within a complex to be crucial components of

bird habitat, greatly influencing the habitat suitability of nearby large wetlands. The

38

study also found that some species, such as the Virginia Rail, are hardly affected by

landscape characteristics at all, but instead habitat suitability is determined by vegetation

characteristics within a wetland.

Urbanization is responsible for fragmentation of many wetland complexes. An

investigation of landscape variables around wetlands found urban development to affect

the wetland bird community (DeLuca, et al., 2004). They found disturbance thresholds at

which the birds responded to. No change in the bird community was seen until this

threshold was reached. Beyond the threshold there were drastic changes in the bird

community. These effects of development were scale-dependent; the threshold response

was only observed when disturbance occurred at the habitat scale. At the watershed scale

the threshold response decayed. Roads were also found to have an effect on the bird

community. The researchers suggest that wetland assessments should include an analysis

of landscape-level characteristics, in addition to the standard within-site characteristics

(DeLuca et al., 2004).

Nest predation creates challenges for all breeding birds, including those in

wetlands. Comparatively lower rates of nest predation in wetlands compared to uplands

may be one of the benefits of nesting there (Picman et al., 1993). A previous study found

water depth to be an important determinant of nest predation rates in wetlands (Stewart,

1999). It is unclear how nest predation rates in wetlands are affected by environmental

features at the habitat and landscape level.

In East Central Minnesota, wetlands are the remnants of the Pleistocene

glaciations. There are many different kinds of wetlands that are classified by their

hydrology, underlying geology, and vegetation characteristics. Swamps, which are

39

dominated by woody vegetation and organic, poorly drained soils, are common in this

area. Marshes are also common. They consist of emergent, non-woody vegetation and

usually occur on sandy soils. There are also bogs with floating mats composed of

partially decomposed organic matter called peat. Some wetlands contain open water all

year, while others are only seasonally flooded (Wovcha et al., 1995).

The purpose of this study was to document the association of different habitat and

landscape features with bird assemblages and nest predation rates in wetlands in and

around the Cedar Creek Natural History Area in East Bethel, Minnesota. The main goals

of the study were to determine the effects of the scale of analysis on the correlations

between bird communities and environmental features, and to determine the effects of

urbanization and agriculture on wetland bird communities. It was hypothesized that the

lowest species richness would be seen in wetlands surrounded by the most urbanization

and agriculture, due to the greater habitat fragmentation and lower habitat connectivity

likely to occur in these areas. Wetlands in more remote areas were expected to show

higher richness and diversity due to greater habitat connectivity. It was also expected

that wetland size and shape, as well as proximity to other wetlands would influence the

composition of wetland bird communities. Nest predation rates were expected to be

higher in sights with little diversity in the vertical structure of the vegetation, due to the

lack of well-concealed locations for nests.

40

Methods

Study Sites

This study was carried out during the summer of 2006 at the Cedar Creek Natural

History Area (CCNHA), a National Science Foundation Long Term Ecological Research

(LTER) site in East Bethel, Minnesota (Figure 1). Twelve wetland sites were selected;

eight on Cedar Creek property, and four in the immediate surrounding area of East

Bethel. An attempt was made to select for heterogeneity among sites in terms of

vegetation, hydrology, and level of development. At each site, a 150 by 75m census plot

was established, with one of the plot edges adjacent to the road or path at the wetland

edge. This plot served as the boundary for the vegetation analysis and the space within

which birds were recorded during surveys.

Bird Surveys

A point-count method was used to survey birds. Surveys were conducted at each

site once a week, between seven and nine a.m. for four weeks between June 29 and July

25, for a total of four surveys at each site. At each site surveys were conducted from two

fixed points along one edge of the 150 by 75m box. Observers stood at each point for ten

minutes and recorded all the birds seen or heard within the site boundaries during that

time. Birds flying over or through the site, clearly without using the space, were not

recorded.

41

Measuring nest predation

Artificial nests made of tennis ball halves with Spanish moss glued to the outside

were used to measure nest predation rates in the eight wetland sites on Cedar Creek

property. Five nests were placed in each site, 1 to 2 meters above the ground in trees or

shrubs. The nests were attached to the vegetation with wire. A quail egg and a clay egg

were placed in each nest (Figure 2). The clay eggs were made of gray clay with black

spots to mimic the quail eggs. The clay and quail eggs were both approximately the size

of warbler eggs. The clay eggs were used in addition to the quail eggs so that different

types of predation marks could be seen. Double incisor marks in clay eggs were evidence

of predation by a mammal, while pierced holes in quail or clay eggs were evidence of

predation by a bird. These marks on the clay egg allowed evidence of predation to be

seen and recorded even if both eggs were still in the nest.

Eggs were placed in the nests and checked every day for three days. They were

checked again after one week, and any remaining eggs were removed. For each nest

checked, the presence or absence of each egg was recorded. For eggs remaining in the

nests, any signs of predation, such as piercing, claw marks, or teeth marks were also

recorded. Predation was measured twice in each site with two sets of eggs.

Predation rates were calculated using the method described by Mayfield (1975).

The probability of egg mortality per day is equal to the number of eggs lost during the

observation period divided by the sum of the number of days that each egg was exposed,

or, egg days.

42

Habitat Analysis

The vegetation within each site was characterized along two 50 meter transects in

each plot. The two transects originated at the two corners of the 150 by 75 meter plot

that were closest to the road or path. Each transect was angled toward the center of the

site at 45 degrees. The length of the transects covered by vegetation of four different

height categories (<1m, 1-2m, 2-5, and >5m) was recorded. The type of vegetation was

also classified as cattail, other graminoid, other herbaceous, moss, trees, or shrubs. Bare

ground, open water, and floating vegetation were also recorded. Each of these substrate

types provides foraging habitat for different types of birds. The percentage of the

transect covered by each vegetation type was calculated, and this number was used as an

estimate of the amount of the wetland site covered by that vegetation type. Because trees

were scattered on the edges of the wetlands, they were not well represented in the

transects. To account for this, the area covered by trees was visually estimated for each

wetland and the percent coverage was calculated from this estimate.

Landscape Analysis

Landscape analyses were conducted at four different scales using Cedar Creek

GIS, Google Earth, and Adobe Illustrator. The total area surveyed in the landscape

analysis was approximately 42 square kilometers. The landscape surrounding each site

was analyzed within 100, 250, 500, and 1000 meters of each site. For the eight sites on

Cedar Creek property, the analysis was conducted in Cedar Creek GIS. For each scale,

the analysis was completed within a circle with that radius around the site. The lengths

of paved and unpaved road within the circle were recorded. The area in square meters

43

and the percentage of the circle covered by wetland, permanent lake, grassland,

woodland, farmland, and development were also recorded. The number of wetland

patches within the circle was counted and the average distance between the study site and

all other wetland patches in the circle was calculated.

Water chemistry data collected from wetlands at CCNHA were also mapped and

compared to patterns of bird distribution. Data on water chemistry were provided by

Martha Phillips, an investigator at CCNHA.

Data Analysis

The birds observed were analyzed by species, foraging guilds, and habitat guilds.

Birds were divided into wetland obligates which nest in wetlands, woodland birds, and

other non-wetland obligate and non-woodland birds. Birds were also divided into seven

foraging guilds; ground and litter foragers, ground probers, foliage gleaners, dabblers and

grazers, excavators, aerial insectivores, and fish eaters, according to the guild

descriptions given by Alcock (2004). Life history information on the Cornell Lab of

Ornithology website’s bird guide (2003) was used to separate the species into guilds.

Species that were only observed once were excluded from the analysis. The purpose of

using guilds was to increase the applicability of the results to other studies. Studies

conducted in other locations will have different species of birds, but will likely have

species that fall into the same guilds as in this study.

Data were analyzed using reciprocal averaging, an indirect gradient analysis

method. Reciprocal averaging is a graphical ordination technique that displays rows and

columns of data as points in a vector space. The distance between the points indicates the

44

strength of the association between them (Davis et al., 2000). Reciprocal averaging

analyses were conducted in PC-ORD. Site coordinates produced from the gradient

analysis were correlated with the habitat and landscape measurements for each site in

Microsoft Excel to determine which environmental variables were associated with the

axes of the indirect gradient analysis at the significance level p < 0.1. The variables

found to be significantly correlated with the axes were used in a canonical

correspondence analysis (CCA) with the significantly correlated bird species.

Data were also analyzed using indirect gradient analysis and canonical

correspondence analysis with wetland bird data collected in a previous study at CCNHA

in 2001 by Aletia Van Brocklin and Louise Bier (Van Brocklin, 2002). Three of the

environmental variables, percent herbaceous vegetation, percent woody vegetation, and

percent open water, were measured in both 2001 and in 2006. In addition, diversity of

these three cover types was calculated in 2001 using the Shannon diversity index. Cover

type diversity was also measured for the 2006 data. These analyses were done to

determine if wetland birds responded to wetland cover diversity in the two years. Water

chemistry data were compared to the ordination plots of the indirect gradient analysis of

species.

Results

Twenty-one of the sixty-six environmental variables measured were found to be

significantly correlated with the axes in the indirect gradient analysis of species (six at the

0.1 level, eight at the 0.05 level, and seven at the 0.01 level). Significant (p < 0.1) results

of the correlations between environmental variables and the X and Y values assigned to

45

the sites by the indirect gradient analysis of birds are shown in Tables 1 and 2. Table 1

shows the environmental variables and bird variable at the habitat level that were

significantly correlated with the axes. The significance levels of the correlations are also

shown.

A total of forty-four species of birds were observed in the twelve sites. Forty of

these species were observed more than one time throughout the survey period. The total

number of birds observed and identified in each site ranged from 38 to 151. The number

of bird species observed in each site ranged from twelve to twenty-five. The four species

with a total of only one individual observed in all surveys were not included in the

analyses. Of the forty species analyzed with indirect gradient analysis, 14 were found to

be significantly correlated with the X or Y axes. Thirty-six species were not significantly

correlated with the axes. The fourteen significantly correlated species and their spatial

relationships to each other and to the axes are shown in Figures 4 and 5. Figure 4 shows

the distribution of species with the X-axis, and Figure 5 shows the distribution of the

species with the Y-axis. The closer two points are to each other, the more similar those

two species are in terms of the sites where they were observed. Similarly, the closer

together the sites are on the plot, the more similar they were in terms of bird composition.

All of the species classified as woodland obligates (green) were located in the top left

quadrant. The two wetland obligates (purple) are in the top two quadrants.

Figure 6 shows the X-axis of the indirect gradient of species with an arrow

indicating the direction of urban development and agriculture. The figure shows that the

Killdeer (Charadrius vociferus), Mallard (Anas platyrhynchos), and Tree Swallow

(Tachycineta bicolor) were positively associated with development and agriculture, while

46

the majority of the species shown are negatively correlated with development and

agriculture. High levels of development and agriculture in the landscape are associated

with a high number of bird individuals at a wetland site, while low levels are associated

with high bird species richness.

Figure 7 shows the ordination plot of the indirect gradient analysis of species.

The figure shows the locations of the study sites in the vector space, in addition to the

bird species. The clustering of species indicates their occurrence in similar habitat

conditions during the surveys. Similarly, the closer together the sites are, the more

similar they are in terms of the bird compositions observed in them.

Foraging Guilds

Table 3 shows the species that comprised the seven foraging guilds. The ground

and litter forager and foliage gleaner guilds were the most represented foraging guilds,

with 10 species from each guild observed. The species in each guild are shown in Table

3. Table 4 shows the correlations of the environmental variables with the X and Y axes in

the indirect gradient analysis of foraging guilds. The table shows that 15 of the

environmental variables were significant at the p < 0.05 or p < 0.01 levels.

Figure 8 shows the first axis of the indirect gradient analysis of foraging guilds.

Three of the guilds, foliage gleaners, ground and litter foragers, and dabblers were found

to be significantly correlated with the axes in this indirect gradient analysis. The dabblers

were positively associated with the axis, while foliage gleaners were negatively

associated. Environmental variables associated with short vegetation and agriculture and

47

urban development in the landscape were positively associated with the X-axis. Ground

and litter foragers were negatively correlated with the Y-axis.

Figure 8 also shows that, like in the ordination of species, the variables associated

with wetland connectivity within 250 meters were positively associated with the first axis

in the ordination of foraging guilds. The amount of surrounding farmland and paved road

were also positively correlated with the axis, both within 100 meters and within 250

meters.

Habitat Guilds

Of the 40 species analyzed, nine were classified as woodland obligates, and seven

as wetland obligates. One species, the Veery (Catharsus fuscescens), was classified as

both a woodland and wetland obligate based on information provided by Cornell Lab of

Ornithology’s online bird guide (Bird Guide, 2003). Table 5 shows the species classified

as wetland and woodland obligates. Table 6 shows the environmental variables that were

significantly correlated with the axes in the indirect gradient analysis of wetland and

woodland obligates. Figures 9 and 10 show the biplots of the canonical correspondence

analyses of the woodland and wetland obligate guilds. The circles on the graphs indicate

the general range of each guild in the vector space. In this ordination, wetland obligates

were negatively correlated with the X and Y axes at the p < 0.05 level. Woodland

obligates were positively correlated with both axes, at a significance level of p < 0.01.

Figure 9 shows that at the habitat level, the two guilds were associated with opposite

vegetation features. Wetland obligates were associated with grasses and vegetation less

48

than one meter in height, while woodland obligates were associated with taller

vegetation.

Figure 10 shows that woodland obligates shown in green were negatively

associated with the X-axis, and most were clustered in the top left quadrant. The

majority of the wetland obligates were also negatively correlated with the X-axis as well,

although some were positively correlated. These species were not grouped as tightly

together as the woodland obligates were. Development, farmland, and number of

wetlands were positively associated with the X-axis, as were wetland obligates.

Nest Predation

Rate of nest predation was not found to be significantly correlated with either of

the axes (p > 0.1), in the indirect gradient analysis. There was also no statistically

significant relationship between nest predation rates and environmental variables (p >

0.1). There was little difference in predation rate between the clay eggs and quail eggs.

However, nest predation rate varied significantly among sites. Table 7 shows the

probability of predation per day on each nest at each site. The results from Trial 1 and

Trial 2 are shown.

Results from the ANOVA test showed that there were variations in the average

rates of nest predation among the sites, at the p < 0.001 level. The predation rates of sites

3, 7, and 8 were significantly higher than the average rates of the other sites. Sites 3 and

8 had higher average rates than site 7 did. Sites 10 and 12 had significantly lower

predation rates than the other sites.

49

Combined 2001 and 2006 results

The indirect gradient analysis of the combined data set showed that 11 birds were

significantly correlated with the axes. These birds and their distributions are shown in

Figure 11. Figure 11 shows the biplot first axis of the ordination plot of the indirect

gradient analysis of species observed in 2001 and in 2006 with the cover diversity

gradient. The graph shows that higher cover diversity was negatively associated with the

X-axis. Of the species significantly associated with the axis, the majority were positively

associated with high cover diversity. Only the Red-winged Blackbird and Black Tern

were associated with low cover diversity.

Water Chemistry

Figure 12 shows a map of CCNHA with the water collection sites and measured

pH and the bird survey sites marked. Only three of the sampled for water chemistry data

were also wetland bird sites. South of Fawn Lake Drive, there was a trend found in the

water pH of the sites measured. Near site 1, pH was 7.085, close to neutral, and had

declined to 4.895 near site 12. In this region of the study area, water becomes more

acidic from north to south. There was no similar trend observed north of Fawn Lake

Drive. For the three bird observation sites where water samples were collected from,

there was a trend between the distribution of sites based on bird species data and water

pH. This trend is shown in Figure 13. The axis shown is the X-axis of the indirect

gradient analysis of species. There is a trend toward higher pH with higher levels of

urban development and agriculture in the landscape.

50

Discussion

Results show that considerable avian use of wetlands is by birds that breed in

other habitats. As Table 4 shows, ten woodland obligates were observed in the wetland

sites. It is not surprising that the presence of these species was associated with the

vertical structural diversity within a site and with the presence of trees in the surrounding

landscape. Twedt et al. (2002) also found that birds that breed in woodlands used

wetlands within a woodland matrix. The presence of woodland birds clearly contributes

to the high species richness in forested wetlands.

Effects of agriculture and urban development

Figures 7 and 8 show that of the fourteen birds significantly correlated with the

axes in the indirect gradient analysis, ten were negatively associated with farmland and

development.

Also in these figures, the association of the Tree Swallow, Mallard, and Killdeer

with farmland, development, and paved road suggests that these species are not

negatively affected by these types of land use in the proximity of their habitats, and

perhaps are even positively affected by them. Mallards were strongly associated with the

X-axis, as were these environmental variables, suggesting that they are able to breed with

the amount of farmland and development currently in and around CCNHA. The thirty-

six species that were not associated with the axes in the indirect gradient analysis are also

not likely affected by the amount of farmland and urban development in the area. These

species were also not associated with within-habitat characteristics, suggesting that they

51

are more generalist species than the fourteen that were significantly correlated with the

axes.

The number of wetlands within 250 meters and the total wetland area within 1000

meters were also positively correlated with this axis. Although it is counterintuitive that

there was more wetland area in developed areas than in remote places, it is possible that

the amount of urban development near CCNHA may currently be low enough that

wetland area has not yet been affected. There could also be constructed ponds and

drainage ditches filled with water in these areas. It is also possible that the low wetland

area and density recorded in undeveloped areas is due to the resolution of the photos used

in the analysis. Patches were typically recorded as wetlands if there was some open

water present. Forested wetlands or seasonally dry wetlands could have been present in

undeveloped areas but were not counted due to their low visibility in the aerial photos.

It is likely that the Mallards are responding to the higher amount of wetland

habitat in developed areas and not specifically to development. However, Picman et al.

(1993) found that dabbling ducks fed on emergent wetland vegetation in the summer and

foraged in agricultural fields later in the year, while remaining near areas with open water

and vegetation. Perhaps for these birds, wetlands in farmland matrices offer ideal habitat

by providing an abundant food source and protection. Perhaps the strong association of

Mallards with farmland, in addition to wetland number and area, indicates that Mallards

are specifically selecting wetlands surrounded by farmland.

Habitat Guilds. Figure 13 shows that as a group, woodland obligates were

negatively associated with farmland and development. As a group, the wetland obligates

52

were not as constrained by environmental variables as woodland obligates were. The

woodland obligates were more negatively associated with urban development, paved

road, and agriculture than wetland obligates, suggesting that they are more sensitive to

increases in farmland and development. However, only the Mallard was positively

associated with farmland and development. The negative association of woodland birds

with urban development, paved road, and agriculture is likely due to the lower density of

trees in these areas. Table 3 shows that many of the woodland obligates are foliage

gleaners or excavators, so the absence of trees would make it more difficult for these

species to find food. Figure 11 shows that foliage gleaners were positively associated

with the area covered by trees within 500 meters and within 1000 meters. This suggests

that woodland birds, which include many foliage gleaners, seek out wooded areas at

broader landscape scales. Perkins et al. (2003) also found that the amount of woodland

present at broad spatial scales was important for some species of woodland birds in

landscapes dominated by agriculture. They found that the Eastern Wood-pewee and

Great Creasted Flycatcher were especially dependent on the amount of woodland within

500 meters of the study sites.

Four of the wetland obligates, the Veery, Swamp Sparrow, Marsh Wren, and

Virginia Rail, were also negatively associated with farmland and development as Figure

10 shows. These results suggest that the breeding habitats of some wetland obligates

could be diminished by the expansion of farmland and development in the area. Of these

species, the Marsh Wren was found to be associated with vegetation 1-2 meters tall and

2-5 meters tall. For this species, the negative association with farmland and development

could be a reflection of the lack of variability in the structure of the vegetation in these

53

areas. Marsh Wrens glean insects off of marsh vegetation (Bird Guide, 2003). Like the

woodland obligates, the Marsh Wren likely can only use wetlands with structural

vegetation that provides its primary food source, although the structure that it requires is

not woody vegetation, like the majority of the foliage gleaners. From these data, it could

be hypothesized that with the expansion of farmland and development, woodland

obligates will first disappear from wetlands, followed by sensitive wetland obligates.

Since there was no development recorded within 100 meters of the wetland sites,

it is unclear how development immediately adjacent to wetlands affects wetland bird

communities. As development likely causes a reduction of variety in the structure of

vegetation at a fine scale and the reduction of woodland density at broader scales, it can

be hypothesized that development adjacent to wetlands would lead to a decrease in the

species richness of birds in the wetland.

Chapman and Reich (2007) found that sensitive native bird species from different

habitat types, woodland, savanna, and grassland, decreased in abundance across a

gradient of urbanization in the Twin Cities area. Results of this study suggest that for

wetland birds as well, abundance could decrease with increasing urbanization. Chapman

and Reich (2007) suggest that there is a disappearance threshold associated with

development, especially for sensitive savanna and grassland birds. The birds would

become less abundant as development intensifies until a threshold is surpassed and

species are eliminated from the region (Chapman and Reich, 2007). There may be a

similar trend for some wetland species as well. DeLuca et al. (2004) found a

development threshold within 500 meters of wetlands for wetland birds in the

Chesapeake Bay area.

54

It is interesting and somewhat surprising that agriculture and development were

associated with the same axes and bird species in all ordination plots. This suggests that

in this area birds using wetlands are responding the same way to adjacent agriculture as

to development. Chapman and Reich (2007) found that bird diversity of upland species

was much higher in agricultural areas near reserves than in developed areas. Another

study at CCNHA also found that upland birds were strongly negatively affected by

surrounding urbanization and roads, but they used agricultural fields as habitat

(Goldsmith, 2007). In contrast, my results suggest that birds using wetlands are

negatively affected by both development and agriculture in the surrounding landscape. It

is possible that the lack of difference in the associations of development and agriculture

with the bird communities was due to the low amount of development in the area

compared to agriculture. Even the sites that were surrounded by the highest amount of

development were still surrounded by even larger areas of agriculture.

Effects of paved road. Paved road was significantly associated with the bird

distribution at multiple landscape scales. The majority of the species that were

significantly correlated with the axes were negatively associated with paved roads.

Findlay and Houlahan (1997) also found that wetland bird species richness decreased

with density of paved roads. In contrast, Findlay and Bourdages (2000) found that

species richness is not very sensitive to paved road density.

Results of this study suggest that paved road density at multiple landscape scales

affects species richness of birds in wetlands, however the mechanism is unclear. It could

be that that, since paved road density is associated with more developed areas, the lower

55

number of species is due to the reduction in woodland area, which was also found to be

associated with development. In this case, paved road and development function as one

environmental variable. However, since the graphs show that paved road and

development are associated differently with the wetland bird distribution, it is likely that

paved roads have effects on the bird community that are different from the effects caused

by development in general.

The mechanism by which the length of paved road affects bird distribution could

differ at the different landscape scales. Paved roads could fragment the landscape and

separate wetland patches, creating challenges for species such as the Virginia Rail, that

don’t move among patches during the breeding season (Bird Guide, 2003). Roads

dividing wetland patches into fragments could reduce the habitat size for these birds, and

possibly limit their access to resources. Paved road immediately adjacent to wetlands

could also cause chemicals and sediments to run into the site. It is unclear how this

would affect the bird distribution. At broader spatial scales, noise pollution caused by

paved road could disturb breeding birds. Findlay and Bourdages (2000) found that there

was a time lag between road construction and visible effects on bird communities. It is

therefore important that studies of the effects of paved roads on wetland communities

occur over a variety of both spatial and temporal scales.

Foraging guilds. Figures 9, 10, and 11 show that the four foraging guilds found to

be significantly correlated with the axes varied widely in their distribution in the vector

space. While foliage gleaners were strongly associated with trees at broad scales and

dabblers were correlated with farmland, development, and short vegetation, Figures 10

56

and 11 show that the ground and litter foragers and ground probers were not strongly

associated with any of the environmental gradients. Ground probers were slightly

positively associated with short vegetation, development, and farmland, while ground and

litter foragers were slightly negatively associated with these features. While it is unclear

from these results what environmental variables the distribution of these groups are

associated with, the figures suggest that they are not strongly affected by farmland and

development in the landscape, but rather, they are more generalists than birds in other

foraging guilds.

While Figures 6 and 7 show that few species were associated with farmland and

development, greater numbers of bird individuals were seen in these sites. It is possible

that this trend is due to sampling bias. Birds are easier to detect and count in open areas.

In sites with high structural diversity most birds were identified by their calls. It is much

more difficult to count birds by this method, and birds present but not calling were not

observed. At site 7 where the fewest birds were seen, almost all identification was by

call. Due to a thick layer of shrubs and trees adjacent to the road, very few birds in the

site could actually be seen.

Wetland connectivity

There was a large effect of scale in this study. Environmental variables were

found to be important at both the habitat and landscape scales. In this study, number of

wetlands within 250 meters, average distance to wetlands within 250 meters, and wetland

area within 1000 meters were found to be correlated with the axes. It is surprising that

number of wetlands and average distance to wetlands were both positively correlated

57

with the X-axis. A high number of wetlands within a landscape suggest high

connectivity of wetlands, while higher distance between wetlands suggests low

connectivity. However, number of wetlands was correlated the p < 0.01 level, while

average distance to wetlands was only correlated at the p < 0.1 level, suggesting that the

number of wetlands is a more important variable. Several other studies have found

wetland connectivity to be important in explaining bird distribution (Fairbairn, 2001;

Haig et al.; 1998, Naugle et al.; 2000). Semlitch and Brodie (1998) found that for species

with home ranges spanning a large area containing many wetlands, the loss of small

wetlands makes travel between larger patches more difficult. Naugle et al. (2001) also

concluded that species that use resources over large spatial scales will be the most

vulnerable to the loss of small wetlands.

Trends in wetland connectivity variables were not uniform across scales. Wetland

density was found to be important only within 250 meters, while wetland area was only

important within 1000 meters. The lack of a clear trend could be partially due to the lack

of independence at some of the sites at the broader scales. For these sites, the area

analyzed within 500 meters and within 1000 meters overlapped, making the analyses for

these sites very similar to each other. These results could also indicate that in this area

the level of connectivity is not the most important feature for determining habitat

suitability for wetland birds. Other habitat and landscape features could be more

important, and the birds may be responding primarily to these features. It is also possible

that connectivity among wetland patches in this area is high enough that the birds are not

significantly affected by varying connectivity among patches. Perhaps, like for

58

development, there is a threshold for loss of connectivity, beyond which birds are

significantly affected, but that threshold has not been surpassed in the CCNHA area.

Effect of wetland size and shape

It was expected that wetland size and shape would have an effect on bird

distribution; however no relationship was found between wetland size, amount of edge,

and the axes in the ordination plots. In contrast, Riffel et al. (2001) found large wetlands

to be important for several bird species. It is possible that area was not found to be

important in this study because the effect was masked by other measured environmental

variables. Perhaps features such as woodland in the proximity of the wetland or

vegetation features within the wetland are more crucial for birds than wetland size. This

would mean that even if a certain wetland size is preferable for a species, it would not be

able to use the site unless another environmental feature is present. These results

question the use of wetland area as the primary criterion for the selection of preservation

sites.

Effect of vertical and horizontal structural diversity

The variables found to be significant at the habitat level were mainly associated

with the vertical structure of the vegetation. Twedt et al. (2002) also found that in

bottomland swamps that had been reforested, colonization by woodland breeding birds

occurred much more in sites with fast-growing, tall vegetation than in sites with shorter

vegetation. This suggests that the presence of some birds is at least partially dependent

on the within-patch structure. While the woodland obligates were mostly found to be

59

positively associated with greater structure within the wetland, some other species were

found to be associated with short vegetation and graminoid vegetation. Several variables

were found to be significant at the landscape level as well. The figures show that most of

the bird species observed were negatively associated with development and agriculture.

Results suggest that to provide habitat for a diversity of bird species that use

wetlands, heterogeneity in the types of wetland patches within a landscape is important.

Some species, like the Mallard and Killdeer were found mostly in sites with open water

and little tall vegetation, while other species were found in sites with more diversity in

the structure of the vegetation. Killdeer are not typically associated with open water

(Bird Guide, 2003), so this relationship is surprising. It is likely that these birds are

responding to the lack of structural vegetation that was found to accompany open water.

Results also suggest that both local and landscape features are important to consider.

Woodland birds were mainly associated with trees at the landscape scales, while the

wrens were also strongly associated with vertical structure within the wetland patch.

DeLuca et al. (2001) also found that environmental variables were important at both local

and landscape levels.

Effect of wetland cover diversity

The 2001 study also found that structural variation within wetlands is important in

explaining bird distribution in wetlands. The analysis of the data compiled from both

years also illustrated the significance of cover diversity. The majority of species that

were significantly correlated with the axes were positively correlated with a high cover

diversity index number. Because the index was calculated using measures of structural

60

diversity, this suggests that more bird species from the two years of data are associated

with areas of high structural diversity. The three birds that were negatively associated

with the diversity index were the Red-winged Blackbird, Black Tern, and Veery. As

terns typically have habitat ranges over large spatial scales and require open water and

open space in the landscape (Naugle, et al., 1999), it is logical that their distribution is not

explained by high within-patch structural diversity. The Red-winged Blackbirds often

breed in areas of high cattail density (Weller, 1999), so a high diversity in the structure of

vegetation would likely not be necessary for their breeding success as long as their

preferred vegetation type is available. However, it is surprising that the Veery was not

found to be positively associated with high structural diversity in this analysis.

Nest predation

It was surprising that, while nest predation rates varied significantly among sites,

predation rates were not significantly correlated with any of the ordination axes or with

any measured environmental variables. Picman et al. (1993) suggest that the main

predators in marshes are Marsh Wrens, however we found no correlation between

predation rate and distribution of the Marsh Wren. The two sites with the highest nest

predation rates were both bogs with floating mats and less structural vegetation than most

of the other sites. It is possible that the high predation rates in these sites were due to the

lack of structure, however associations with these variables were not statistically

significant due to the small number of sites.

It is also possible that nest predation rates varied significantly in features at scales

smaller than those that were measured. A single wetland patch was the smallest scale

61

that variables were measured at. However, other studies have found nest predation rates

to vary among locations within a single wetland (Picman et al., 1993; Stewart, 1999). It

is possible that differences in predation rates could be explained by differences in

features in the immediate vicinity of the nests, which were not measured. For future

studies, I recommend measuring environmental conditions at much finer scales, as well

as at broad scales to determine the scale at which nest predators respond.

Water chemistry and hydrologic cycles

Figure 17 shows a clear trend in pH, from near neutral to more acidic going south

from Fawn Lake Drive. While pH was not measured at the sites where bird surveys were

conducted, this trend suggests that water at sites 1 and 11 are close to neutral, with water

at site 2 being more acidic and water at sites 3 and 12 still more acidic. Figure 4 shows

that there is a slight trend in the location of these sites on the graph of the indirect

gradient analysis of species.

There was a trend in acidity among the three sites where both water data and bird

data were collected. Figure 16 shows that acidity decreases along the first axis in the

ordination of species. Following this trend, Figure 4 suggests that woodland obligates are

associated with sites with acidic water, while wetland obligates are associated with water

with more neutral pH. To determine if there is a significant relationship between bird

distribution and water pH, in future studies pH should be measured at the sites where bird

data is collected. Sites 3 and 12 were also very similar in their vegetation compositions,

as were sites 1 and 11, which likely has a larger effect on the distribution of bird species.

However, the pH of the water and soil in the site could play a large role in determining

62

the vegetation composition. Other features of water chemistry, such as salinity should

also be measured. Salinity could affect vegetation growth and subsequently, bird use in

wetlands. The salinity of wetland water could also be affected by land use in the

surrounding landscape, especially by agricultural uses.

Although water level was not directly measured in this study, changes in the water

levels at some of the wetland sites were observed. Some sites that had standing water in

the initial surveys were completely dried out by the end of the study. Changes in the bird

communities at some of the sites over this period were noticed as well. The Virginia

Rails and Spotted Sandpipers were only seen in the last two weeks of the study. In

contrast, Veerys were observed in the first week of the study in the majority of the sites,

but was observed in none of the sites during the last two weeks. I recommend that in

future studies water level be monitored throughout the course of the observation period

so that the effects of hydrological fluctuations can be observed.

Conservation Implications

Results of this study have implications for wetland conservation, restoration, and

construction projects. Most importantly, results show that it is necessary to consider

environmental variables at multiple scales when planning wetlands projects. Conserving

and restoring appropriate habitat for wetland birds is especially important, as Valiela and

Martinetto (2007) found that wetland birds have decreased in abundance by 34 percent

over the past 50 years.

Results of the habitat-level analyses suggest that within a patch bird species

richness can be maximized by the inclusion of horizontal and vertical structural variation

63

in the vegetation. Vegetation two meters or taller may attract woodland bird species.

Vegetation taller than 1 meter can also provide habitat for wetland obligates such as the

Marsh Wren and Veery that use habitats with some vertical structure. However,

landscape features are important to consider for the distribution of birds within the

wetland patch as well. While diversity in the structure of vegetation may allow a variety

of birds to use the site, results suggest that woodland in the proximity of the wetland is

necessary woodland birds to use the wetland. However, a large density of woodland in

the surrounding area may limit the ability of some species, such as Mallards, to use the

site. These results suggest that sites for restoration should be carefully chosen based on

the characteristics of the surrounding landscape and the goals for restoration or

construction. To provide habitat for the greatest possible diversity of birds, sites in the

proximity of woodland should be selected. However, more open areas should be selected

to provide habitat for some wetland species.

Campbell and Ogden (1999) suggest that cattails, bulrushes, and common reeds

are the vegetation types typically used in constructed and restored wetlands because of

their vigorous growth and wide availability. While these species may me ideal for the

desired water filtering and storage functions (Campbell and Ogden, 1999), results of this

study suggest that wetlands dominated by these species may not provide ideal habitat for

a wide diversity of birds because they provide very little variety in both horizontal and

vertical structure. Engelhardt and Richie (2001) also hypothesized that with higher

vegetation species richness more wildlife could be supported in wetlands. They found

higher overall biomass in wetlands with a greater diversity of vascular plants (Engelhardt

and Ritchie, 2001). If one of the goals of wetland construction or restoration is high

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biodiversity of birds, then plans should involve planting species to provide vertical

structure in the vegetation.

My results suggest that, in contrast to what Findlay and Bourdages (2000) found,

birds can be indicators of environmental changes in wetlands due to increasing urban

development, paved road lengths, and agriculture in the landscape. While Findlay and

Bourdages (2000) concluded that birds are not very sensitive to construction of paved

roads, this study suggests that birds are responding to paved road in the proximity of the

wetlands, as paved roads were associated with low bird species richness.

From the results of this study, it can be concluded that in wetlands, environmental

variables at both habitat and landscape scales are important for explaining the distribution

of birds. It appears that high bird species richness is dependent on the structure of the

vegetation within the habitat and on woodland in the surrounding landscape. Birds using

wetlands near CCNHA seem to be responding the same way to agriculture as to

development. I hypothesize that if agriculture and development continue to expand in

this region, the bird species richness in many of the wetlands will decrease primarily due

to loss of wooded areas.

65

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Variable X Y Significance Graminoid vegetation + p < .1 Snags + p < .05 Vegetation < 1m tall + p < .05 Vegetation 1-2m tall - p < .05 Vegetation 2-5m tall - p < .1 Number of birds + p < .05

Table 1: Habitat variables and bird variables correlated with the X and Y axes. The table shows that 4 environmental variables and 1 bird variable were correlated with the X-axis in the indirect gradient analysis. Only 1 environmental variable was correlated with the Y-axis. Two of the variables were significantly correlated at the p < 0.1 level, and 4 were significant at the p < 0.05 level.

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Scale Variable X Y Significance100m

Farmland + p < .01 Paved road + p < .1 250m

Farmland + p < .01

Number of wetlands + p < .01

Distance to wetlands + p < .1

Trees - p < .1 500m

Farmland + p < .01 Development + p < .01 Paved road + p < .05 Trees - p < .05 1000m

Farmland + p < .05 Development + p < .01 Paved road + p < .01 Wetland area + p < .1

Trees - - p < .05, p < .1

Table 2: Landscape variables significantly correlated with the axes in the indirect gradient analysis of species. The table shows that a total of 15 variables at landscape scales were found to be significantly correlated with the axes. Two variables were significant within 100m, four withing 250m and within 500m, and five within 1000m.

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Ground/Litter Forager

Ground Prober

Foliage Gleaner Excavator

Aerial Insectivore Dabbler

Fish Eater

Veery Swamp Sparrow

Common Yellowthroat

Downy Woodpecker Tree Swallow Duck

Great Blue Heron

Red-winged Blackbird

American Robin House Wren

White-breasted Nuthatch Barn Swallow

Canada Goose

Green Heron

American Goldfinch

Spotted Sandpiper Marsh Wren

Eastern Wood Pewee

Ovenbird Killdeer Gray Catbird Eastern Kingbird

Lark Sparrow Northern Flicker Blue Jay

Eastern Phoebe

Song Sparrow Virginia Rail

Scarlet Tanager

Great Crested Flycatcher

Vesper Sparrow Cedar Waxwing

Alder Flycatcher

Brown-headed Cowbird

Black-capped Chickadee

Eastern Bluebird

Mourning Dove Rosebreasted Grosbeak

American Crow Northern Cardinal

Table 3: Seven observed foraging guilds. The ground/litter forager and foliage gleaner guilds were the most represented guilds with 10 species in each. The excavator, dabbler, and fish eater guilds each had two species, and therefore were the least represented guilds in terms of number of species.

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Scale Variable X Y Significance Habitat

height <1m + p < .05 height 1-2m - p < .1 100m

Farmland + p < .05 250m

Farmland + p < .05

Number of wetlands + p < .05

Distance to wetlands + p < .05

500m Farmland + p < .01

Development + p < .01 Paved road + p < .05 Trees - p < .05 1000m

Farmland + p < .1 Development + p < .01 Paved road + p < .05

Table 4: Environmental variables significantly correlated with the axes in the indirect gradient analysis of foraging guilds. While 15 variables were significantly correlated with the axes in the indirect gradient analysis of species, 13 were significant in the analysis of foraging guilds.

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Wetland Woodland

Marsh Wren Ovenbird Swamp Sparrow Scarlet Tanager

Mallard Black-capped Chickadee

Green Heron Rose-breasted Grosbeak

Virginia Rail Downy Woodpecker

Veery White-Breasted Nuthatch

Eastern Wood Pewee

Great Crested Flycatcher

Veery

Table 5: Wetland and woodland obligates observed. A total of six wetland obligates and nine woodland obligates were observed in the study.

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Scale Variable X Y Significance Habitat

Graminoid vegetation + p < .1

height <1m + p < .05 height 1-2m - p < .1 height 2-5 - p < .05 100m

Farmland + p < .05 250m

Farmland + p < .01

Number of wetlands + p < .01

Distance to wetlands + p < .1

500m Farmland + p < .01

Development + p < .05 1000m

Development + p < .05 Table 6: Environmental variables significantly correlated with the axes in the ordination of woodland and wetland obligates. A total of 11 variables were significantly associated with the axes in the ordination of woodland and wetland obligates.

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Trial 1 Site 1 Site 2 Site 3 Site 7 Site 8 Site 10 Site 11 Site 12

Nest 1 0 0 0.5 0.5 1 0.5 0.142857 0Nest 2 0 1 1 0 1 0.142857 0.142857 0.5Nest 3 0.142857 0 1 0.5 1 0 0 0Nest 4 0.142857 0.5 1 1 1 0 0.142857 0Nest 5 0 1 1 0 0 0

Trial 2 Site 1 Site 2 Site 3 Site 7 Site 8 Site 10 Site 11 Site 12

Nest 1 1 0 1 0.142857 0.142857 1 0 0Nest 2 0.333333 0.5 1 0.5 0.5 1 0.142857 0Nest 3 0.142857 0.333333 0.5 0.5 0.142857 0 0 0Nest 4 0 0.142857 0.5 1 0.142857 0 0 0.142857Nest 5 0 1 0.142857 0 0 0

Tables7a and 7b: Probabilities of nest failure per day in Trials 1 and 2. The tables show the probabilities of nest failure that were calculated for each nest in each site using the method described by Mayfield (1975). Nests were considered failures only when there was evidence of predation on both the quail and clay eggs.

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Figure 2: Quail and clay eggs in an artificial nest. The nests were placed in trees 1-2 meters above the ground at the edge of the wetland sites. A quail egg and a clay egg were placed in each nest. Photo by author.

Figure 3: Landscape analysis within 500m of Site 2 using Cedar Greek GIS. The green triangle shows the approximate location of the sample plot within the wetland. The yellow circle has a radius of 500 meters with the center the triangle. The red lines stretch from the triangle to other wetlands within the circle. The length of these lines were averaged to give an average distance from the study site to other wetlands as a measure of wetland connectivity.

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Graminoid vegetationVegetation < 1m tall Farmland (100, 250, 500, 1000m) Urban development (500,1000m) Paved road (100, 500, 1000m) Number of wetlands (250m) Distance to wetlands (250m) Wetland area (1000m)

Vegetation 1-2m tallVegetation 2-5m tall Trees (250, 500, 1000m)

KILL MALL TRES RWBL VESP HOWR

DOWO

WBNU

BCCH MAWR

SOSP

EAPH COYE

MODO

X-axis

Woodland Obligate Wetland Obligate

Other

Figure 4: X-Axis of the indirect gradient analysis of species with environmental variables significantly correlated with the axes. The axis represents a gradient in the environmental features. The variables listed on the right side of the axes are positively associated with that end of the axis and the species on that end For example, a high abundance of vegetation 1-2m tall was associated with the left end of the axis and with the presence of the House Wren, and a low abundance vegetation 1-2m tall with the right end of the axis and with the Killdeer. High graminoid abundance was associated with the right end of the axis, and low abundance with the left end of the axis. Species labeled in green are woodland obligates and those labelled in purple are wetland obligates. HW=House Wren, DW=Downy Woodpecker, WB=White-breasted Nuthatch, CH=Black-capped Chickadee, MW=Marsh Wren, EP=Eastern Phoebe, CY=Common Yellowthroat, MD=Mourning Dove, SS=Song Sparrow, VS=Vesper Sparrow, RW=Red-winged Blackbird, TS=Tree Swallow, ML=Mallard, KD=Killdeer.

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WBNU

MAWRHOWR DOWO

KILL

BCCH

MALL

COYE

TRES

EAPH

RWBC

VESP

SOSP

MODOY-axis

High area trees (1000m)High cover snags

Woodland Obligate Wetland Obligate

Other

Figure 5: Y-axis of the indirect gradient analysis of species with environmental variables significantly correlated. A high abundance of snags was associated with the left end of the axis and with the presence of the House Wren, and a low abundance of snags with the right end of the axis and with the Mourning Dove. High tree abundance within 1000m was associated with the right end of the axis, and low abundance with the left end of the axis.

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Urban development and agriculture

Woodland Obligate Wetland Obligate Other

KILL MALL TRES RWBL VESP HOWR

DOWO

WBN

BCCH MAWR

SOSP

EAPH COYE

MODO

X-axis

Figure 6: Direction of development in the ordination plot of species. The red arrow shows the increase in urban development and agriculture toward the right end of the axis. A high number of bird individuals is associated with high development and agriculture, while high species richness is associated with low development and agriculture.

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1

23 4 5

6

7

8

9

1011

12

Mallard

NuthatchDowny

Chickadee

Marsh Wren

Dove

Killdeer

Tree Swallow

Red-winged

House Wren

Yellowthroat

Phoebe

Song Sparrow Vesper Sparrow

X-axis

Y-axis

Figure 7: Ordination plot of bird species. The plot shows that 14 bird species that were significantly correlated with the X and Y axes. The species labeled in green are woodland obligates, the purple are wetland obligates, and the black are generalists. The triangles represent study sites. The spatial relationship on the graph indicates the similarity of the species in terms of the locations where they were observed.

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Foliage Gleaner DabblerX-axis

Vegetation < 1m tall

Farmland (100, 250, 500, 1000m)

Urban development (500, 1000m)

Paved road (500, 1000m)

Number of wetlands 250m

Average distance to wetlands 250m

Vegetation height 1-2m

Figure 8: X-axis of the indirect gradient analysis of foraging guilds with significantly correlated environmental variables. Foliage gleaners and dabblers were the two guilds significantly correlated with the X-axis.

85

1

23

45 67

8910

11

12Graminoidheight <1m

height 1-2m

height 2-5mDowny Nuthatch

Grosbeak

Ovenbird

Flycatcher

Pewee

Chickadee

Tanager

Marsh Wren

Swamp Sparrow Mallard

Virginia Rail

Green Heron

Veery

X-axis

Y-axis

Figure 9: Canonical correspondence analysis of species in woodland and wetland habitat guilds with habitat-level variables shown as vectors. The woodland obligates are labelled in green and the wetland obligates are purple. The Veery was classified as both a woodland and wetland obligate. The circle on the left side of the graph represents the range of the woodland obligates, and the circle on the right side represents the range of wetland obligates. The vectors show that vegetation heights 1-2m and 2-5m were negatively associated with the X-axis, while vegetation height < 1m and graminoid vegetation were positively associated with the axis.

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# wetlands 250mDevelopment 500mFarmland 250mFarmland 100m

Development Farmland 500m

Downy Chickadee

PeweeOvenbir

Nuthatch

Tanager

Grosbeak

Flycatcher

Marsh WrenSwamp Mallard

Green Heron

Virginia Rail

Veery

Y-axis

Figure 10: Canonical correspondence analysis of species in woodland and wetland habitat guilds with landscape-level variables shown as vectors. The small circle in the top left quadrant represents the range of the woodland obligates, and the large circle represents the range of the wetland obligates. All six significant landscape-level variables are positively correlated with the X-axis.

87

TernRed-wingedVeeryBarn Swallow

Vesper

Goldfinch

Catbird

Yellowthroat

Mallard KilldeerTree Swallow

High wetland cover diversity

Low wetland cover diversity

Figure 11: First axis of the CCA of species observed in 2001 and 2006 with the Shannon diversity index variable. The Shannon diversity index was calculated using the cover types herbaceous vegetation, woody vegetation, and open water. H’ is negatively associated with the axis.

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W50

W48

W57 W56

F84

W24

W47

W28

W16

W25

W15

P67

W75

12

3

2

111

8

7

10

4.895

5.565

5.9745

6.1875

5.94

7.085 6.27

5.475

5.32

5.2025

6.305

5.715

5.5525

pHBird sitesWater samples

Figure 12: pH of wetland water sites in CCNHA. The green arrow shows a pH gradient south of Fawn Lake Drive. The water samples get more acidic going southwest from the road. There was no similar trend north of Fawn Lake Drive. Water samples were only collected from three of the sites where bird surveys were conducted.

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Site 7 Site 12 Site 1

5.2 5.6 6.3

Urban development and agriculture

X-axis

Figure 13: First axis of indirect gradient analysis of species with the three sites with pH measurements shown. The pH of the water at these sites is shown in red. Water acidity is negatively associated with the axis. The water approaches neutral pH in the positive direction.