Salinity and Population Parameters of Bald Eagles (Haliaeetus Leucocephalus) in the Lower Chesapeake Bay

Salinity and Population Parameters of Bald Eagles (Haliaeetus leucocephalus) in the LowerChesapeake Bay (Salinidad y Parámetros Poblacionales de Haliaeetus leucocephalus en la Bahia deChesapeake)Author(s): Bryan D. Watts, A. Catherine Markham, Mitchell A. ByrdSource: The Auk, Vol. 123, No. 2 (Apr., 2006), pp. 393-404Published by: University of California Press on behalf of the American Ornithologists' UnionStable URL: http://www.jstor.org/stable/4090669 .Accessed: 19/10/2011 12:56

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The Auk 123(2):393-404, 2006 ? The American Ornithologists' Union, 2006. Printed in USA.

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SALINITY AND POPULATION PARAMETERS OF BALD EAGLES (HALIAEETUS LEUCOCEPHALUS) IN THE LOWER CHESAPEAKE BAY

BRYAN D. WATTS,1 A. CATHERINE MARKHAM, AND MITCHELL A. BYRD

Centerfor Conservation Biology, College of William and Mary, Williamsburg, Virginia 23187, USA

ABSTRACT.-We evaluated the relationship between salinity and Bald Eagle (Haliaeetus leucocephalus) population parameters using 26 years of survey data for the lower Chesapeake Bay. Tidal tributaries within the study area were stratified according to the Chesapeake Bay Program's segmentation scheme, and segments with the same salinity classification were considered spatial replicates. Salinity categories included tidal fresh, oligohaline, mesohaline, and polyhaline. Four parameters- colonization rate, nesting density, projected carrying capacity, and productivity- were derived from nesting data within each shoreline segment and compared across the salinity gradient. The study-wide Bald Eagle population is exhibiting exponential growth, with an average doubling time of 7.9 years. All population parameters showed significant directional variation with salinity. Average population doubling time for tidal fresh reaches was <6 years, compared with >16 years for polyhaline areas. Current Bald Eagle nesting density is negatively related to salinity and varies by a factor of 4 across the gradient. Comparison of current densities with projected carrying capacity suggests that these differences will be stable or increasing as the geographic areas approach equilibrium densities. We suggest that fisheries within lower saline reaches, including spring spawning runs of anadromous Clupeidae (shad and herring), are the most likely explanation for salinity effects. Observed distribution patterns suggest that lands along low-salinity waters are the core of the Bald Eagle nesting population within the lower Chesapeake Bay and should be the focus of long-term programs designed to benefit nesting eagles. Received 23 October 2003, accepted 15 July 2005.

Key words: Bald Eagle, breeding population, Chesapeake Bay, fisheries, Haliaeetus leucocephalus, productivity, salinity.

Salinidad y Parametros Poblacionales de Haliaeetus leucocephalus en la Bahia de Chesapeake

RESUMEN.-Evaluamos la relacion entre la salinidad y parametros poblacionales del aguila Haliaeetus leucocephalus utilizando datos de muestreos realizados a lo largo de 26 afios en la parte baja de la Bahia de Chesapeake. Los tributarios afectados por la marea en el area de estudio fueron clasificados de acuerdo al programa de segmentacion de la Bahla de Chesapeake y los segmentos con la misma clasificacion de salinidad fueron considerados replicas espaciales. Las categorias de salinidad fueron: agua dulce con poca influencia de la marea, oligohalina, mesohalina y polihalina. Se derivaron cuatro parametros - tasa de colonizacion, densidad de nidos, capacidad de carga proyectada y productividad-a partir de datos de nidificacion en cada segmento de la costa y se compararon a traves del gradiente de salinidad. En el area de estudio la poblacion de H. leucocephalus exhibe un crecimiento exponencial, con un tiempo promedio de duplicacion de 7.9 afios. Todos

1E-mail: [email protected]

393

394 WATTS, MARKHAM, AND BYRD [Auk, Vol. 123

los parametros poblacionales mostraron variacion direccional con la salinidad. El tiempo promedio de duplicacion poblacional en las areas de menor influencia de la marea fue < 6 anios, en comparacion con >16 afios para las areas polihalinas. La densidad actual de nidos de H. leucocephalus se relaciona negativamente con la salinidad y varia en un factor de 4 a traves del gradiente. La comparacion entre las densidades actuales con la capacidad de carga proyectada sugiere que estas diferencias seran estables o aumentaran a medida que las areas geograficas alcancen densidades en equilibrio. Sugerimos que la disponibilidad de peces en areas de baja salinidad, incluyendo la oviposicion de primavera de peces anadromos de la familia Clupeidae (arenque y sabalo), es la explicacion mas probable para los efectos de la salinidad. Los patrones de distribucion observados sugieren que las tierras a lo largo de aguas con baja salinidad son el area nuicleo de la poblacion nidificante de H. leucocephalus en la parte baja de la bahia de Chesapeake y deberian ser el foco de programas a largo plazo disefiados para beneficiar a las aguilas nidificantes.

A CENTRAL PROBLEM in ecology is in understanding why the density of a given species varies from one place to another. Ecologists and wildlife managers have spent considerable time and effort identifying factors that contrib- ute to spatial variation in animal numbers and understanding how they operate. For many species of conservation concern, such understanding is essential to effective management. Largely because of their conservation status, Bald Eagles (Haliaeetus leucocephalus) have been studied extensively over the past 30 years. Factors such as human disturbance (Gerrard et al. 1985, Buehler et al. 1991, McGarigal et al. 1991), environmental contaminants (Bowerman 1993, Wiemeyer et al. 1993), and resource limitation (Steidl et al. 1997, Elliott et al. 1998, Anthony 2001) have been suggested to constrain Bald Eagle populations rangewide and, under certain circumstances, to result in spatial variation in nesting density. Resource limitation, in the form of prey availability, has received particular attention from researchers as a mechanism capable of influencing Bald Eagle distribution during both the nesting (Swenson et al. 1986, Hansen 1987, Dzus and Gerrard 1993) and nonnesting (Hunt et al. 1992, Restani et al. 2000) periods.

Like many populations throughout North America, the population of Bald Eagles in the Chesapeake Bay declined dramatically throughout most of the 20th century (Byrd et al. 1990). Although many factors contributed to this decline, accumulation of biocides is widely believed to have been largely responsible for the final and deepest phase of the decline (Wiemeyer et al. 1984, Byrd et al. 1990). The population reached an all-time low of 77 pairs

in the 1970s. Since the early 1980s, the nesting population has increased exponentially, with an average doubling time of between seven and eight years (M. A. Byrd et al. unpubl. data). Nesting Bald Eagles have returned in numbers to many tributaries of the Bay and have re- occupied numerous historical territories documented prior to the population low. However, settlement has not been uniform throughout the region. Nesting densities have reached high levels within some geographic areas, whereas other areas support no nesting activity or only widely scattered pairs. There have been no studies investigating factors responsible for the spatial pattern of recolonization.

The Chesapeake Bay is the largest estuary in the United States and one of the most produc- tive aquatic ecosystems in the world. Unlike the open ocean, where salinity remains constant over vast expanses, estuaries are transitional environments where salinity varies between fresh water and seawater, often over relatively short distances. Salinity is one of only a few abiotic factors that either directly or indirectly influence ecosystem-level processes such as primary productivity, respiration, nutrient cycling, and energy flow (Odum 1988). Salinity tolerances vary widely among aquatic organ- isms. Consequently, salinity is one of the most important determinants of how aquatic animals and plants are distributed throughout an estuary. Much less is known about the influence of salinity on the distribution of terrestrial animals that depend on aquatic food chains. Our objective here was to examine the relationship between nesting Bald Eagles and salinity within the lower Chesapeake Bay.

April 2006] Salinity and Bald Eagles 395

METHODS

The study included the tidal reaches of the lower Chesapeake Bay and the lower Delmarva Peninsula in Virginia (Fig. 1). This location has a steep salinity gradient that is replicated within different tributaries, making it an ideal place to investigate salinity effects on Bald Eagle population parameters. Salinity varies from fresh water near the fall line to full-strength (32 ppt) seawater near the mouth.

The entire study area has been systematically surveyed for nesting Bald Eagles since spring

1977 via a standard two-flight approach (Fraser et al. 1983). The first flight is conducted between late February and mid-March to locate nesting territories. We use a high-wing Cessna 172 aircraft to systematically overfly the land surface at an altitude of -100 m to detect Bald Eagles and nests. The aircraft is flown between the shoreline and a distance of 1-3 km to cover the most probable nesting locations. Nests detected are plotted on 7.5-min topographic maps and given unique alphanumeric codes. Each nest is examined to determine its condition and status. A nesting territory is considered to be "occupied"

Map of study area and salinity zones (Lower Chesapeake Bay)

Legend / Tiai fresh

Oligohaline Mesohaline

Polyhaline 4 ~AI

SI1904.Q44 W 0 Kilomdete ~~~~~~~~~~~~~~~~~~... ........... .... ......... ..

FIG. 1. The Bald Eagle study area in the lower Chesapeake Bay, showing the location of salinity zones according to the Chesapeake Bay salinity segmentation scheme. Areas indicated are 3-km buffer areas inland of the shorelines. Inset map indicates the location of the study area on Virginia's coastal plain.


if a pair of birds is observed in association with the nest and there is evidence of recent nest maintenance. The second survey flight is conducted from late April through mid-May to check occupied nests for productivity and to recheck occupied territories for nesting activity. A plane is flown low over the nest, allowing observers to examine nest contents. Number of nestlings observed is recorded, along with their estimated ages.

Previous authors have outlined several potential sources of bias inherent in the two- flight survey for raptor populations (e.g. Fraser 1978, Steenhof and Kochert 1982, Fraser et al. 1984, Steenhof 1987). One such source arises from pairs that lay eggs but fail prior to the first flight. This bias both underestimates the nesting population and inflates the per-capita reproductive rate. Here, we use the number of territories determined to be occupied, rather than the number of nests with eggs, to represent the nesting population. Although this parameter is subject to similar concerns, we believe that it is more robust with respect to this source of sampling error. A second source of error stems from using a single productivity flight within an asynchronous population. When used alone, a single survey records nestlings from a cross-section of ages and assumes no mortality. The error associated with this assumption overestimates fledging success, because many young nestlings are unlikely to survive to fledging age. One approach used to overcome this error is to use a nest-survival model (e.g. Mayfield 1961, Dinsmore et al. 2002, Shaffer 2004) to correct for mortality. We have not corrected for this source of error. Our primary interest here is to examine spatial variation in nesting density and nestling production. Toward that end, we are interested in relative differences between geographic areas. However, if there is a consistent difference in nesting phenology with geography, a single survey might result in a geographically based bias in this source of error. We used three years (2000-2002) of nesting phenology data to evaluate this possibility. Nesting phenology showed no significant variation with salinity. However, there was a significant shift in phenology with latitude. Pairs within the northernmost locations of the study area nested approximately five days later compared with those in the southernmost locations. In acknowledgment of

this pattern, we have always begun the survey in the south and worked northward to accom- modate this temporal bias.

To evaluate the relationship between salinity and the Bald Eagle population, we subdivided the study area into discrete salinity zones according to the Chesapeake Bay Program's (Environmental Protection Agency) segmentation scheme (Data Analysis Work Group 1997). This scheme is comparable with the well-known Venice System (Anonymous 1959) and breaks the salinity gradient within the bay into four categories. Categories include tidal fresh (TF = 0- 0.5 ppt salinity), oligohaline (OH = 0.5-5.0 ppt), mesohaline (MH = 5.0-18.0 ppt), and polyhaline (PH = 18.0-30.0 ppt). After creating a digital map of all Bald Eagle nest locations coded between 1977 and 2002 (n = 873), we stratified nests according to salinity category and shoreline segment by overlaying a geographic information system (GIS) map of the segmentation scheme (Fig. 1). Population parameters based on the set of associated nests and nesting results were derived for each shoreline segment. We used ARCVIEW, version 3.2 (Environmental Systems Research Institute, Redlands, California) for all digital manipulation.

We analyzed the relationship between salinity and population parameters using a simple one-way approach, where salinity (four levels) was the single factor and segments with the same salinity classification were considered spatial replicates. Sample sizes were 4, 4, 6, and 3 for TF, OH, MH, and PH, respectively. Four population dimensions were considered, including colonization rate, current nesting density, projected carrying capacity, and productivity. Because salinity segments varied in extent, the nesting population was standard- ized within segments, using shoreline length, and expressed as densities (unit = n per 10 km) for comparison. Nesting density was compared between salinity zones using data from 2002. Productivity was evaluated in two ways: productivity density (unit = nestlings per 10 km) and per-capita productivity (nestlings per nesting attempt). Nesting data from 2002 were used for productivity comparisons. Average parameters were compared between salinity zones using a one-way analysis of variance (ANOVA), with salinity as the grouping parameter. Tukey's HSD test was used for pairwise, post-hoc comparisons between salinity categories.


Colonization rates within salinity zones were expressed using the average time (in years) required for the nesting population to double in size (tdouble). Doubling time was calculated using the growth equation N, = N0ert where Nt is the population size in 2002, No is the population size in 1977 or the year the segment was first colonized, e is the base of the natural logarithm, r is the intrinsic rate of increase, and t is the time interval between population estimates. With this configuration, tdouble = ln(2)/r.

We estimated the number of nesting pairs expected at carrying capacity (K) for each shoreline segment by fitting population growth to a logistic curve based on the number of occupied territories found between 1977 and 2002. Morisita (1965) showed that when a population is growing exponentially, (N,+1 - Nt)/N, is linear on Nt+1. A regression of this relationship has the form (Nt+1 - Nt)/Nt = a - bN,+1 where a = er - 1 and b = a/K (Caughley 1977). The intrinsic rate of increase (r) and carrying capacity (K) can be derived for a population by substituting the slope and intercept of the regression relationship into the appropriate equations. This approach assumes that the population is growing exponentially toward an equilibrium density and is subject to the simplifying assumptions of the logistic. Our interest in using this approach here is not to determine specific equilibrium population sizes but instead to estimate whether differences between populations occupying different salinity zones will converge or diverge as they approach capacity.

RESULTS

From 1977 through 2002, the Bald Eagle nesting population within the study area increased

nearly 10-fold from 31 to 305 pairs. The overall population is exhibiting exponential growth, with an average doubling time of 7.9 years. The growth is broadly distributed, and increases in nesting pairs were documented along all shoreline segments examined. However, neither the rate of population growth nor the rate of nestling production has been uniform across the area (Fig. 2). Salinity appears to be one factor contributing to spatial variation in the trajectory of nesting pairs along different water reaches. All population dimensions that we quantified showed a statistically significant response to salinity (Table 1).

Colonization rate (the inverse of doubling time) exhibited a linear decrease with increasing salinity (Fig. 3A). Average doubling time for populations within tidal fresh reaches was <6 years, compared with >16 years for those within polyhaline reaches. Post-hoc comparisons revealed a transition in groupings across the salinity gradient where none of the classes differed significantly from their nearest neighbor, but all differed from their next nearest neighbor.

Nesting density was negatively related to salinity (Fig. 3B). During the 2002 nesting season, average density was more than 4-fold higher for shoreline segments along tidal fresh reaches than shorelines along polyhaline reaches. Post-hoc comparisons suggest that there is a statistical break along the salinity gradient between OH and MH categories. Examination of the density pattern over the 26-year study period (Fig. 2) reveals a divergence between these two groups that appears to accelerate during the 1990s. The underlying cause of this divergence is not apparent. However, the result is that a disproportionate number of the

TABLE 1. Results of one-way ANOVA for average Bald Eagle population parameters. Factor tested is salinity with levels including tidal fresh, oligohaline, mesohaline, and polyhaline.

Source df SS MS F P

Doubling time a 3 and 13 203.771 ? 87.821 67.923 ? 6.765 10.055 <0.01 Nesting densityb 3 and 13 4.644 ? 2.121 1.548 ? 0.163 9.487 <0.01 Carrying capacity C 3 and 13 9.241 ? 5.859 3.080 ? 0.451 6.834 <0.01 Productivity densityd 3 and 13 0.105 ? 0.030 0.035 ? 0.002 14.99 <0.01 Per-capita productivitye 3 and 13 1.115 ? 4.804 0.372 ? 0.083 4.49 <0.01

aAverage time in years for the number of nesting territories to double (1977-2002). bNesting territories per 10 km of shoreline (2002). cNesting territories per 10 km of shoreline (projected carrying capacity). dNestlings produced per 10 km of shoreline (2002). eNestlings per nesting attempt (2002).


Nesting density by salinity

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FIG. 2. Temporal pattern in Bald Eagle nesting density and productivity from 1977 through 2002. Density of (A) nesting pairs and (B) productivity are stratified according to salinity zone.

documented nesting pairs were along the lower saline reaches. This pattern is reflected in the parallel relationship between productivity density and salinity (Fig. 2B).

As with the other population parameters, carrying capacity was significantly and negatively

related to salinity (Fig. 3C). The projected capacity suggests that the overall population had achieved -70% of capacity in 2002. In 1977, when the survey began, the overall population was probably at <7% of capacity. Further examination of capacity patterns reveals variation


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Bald Eagles within the study area. Values are presented by salinity zone for (A) doubling time, (B) 2002 breeding density, and (C) nesting density at carrying capacity. Boxes and whiskers represent ?1 SE and ?1 SD about the mean, respectively. Letters provided above values indicate results of post-hoc comparisons. Categories with the same letter designations were not statistically distinguishable.

between salinity categories in terms of current proximity to expected capacity. Values by category are 70.0, 73.0, 74.2, and 67.8% for TF, OH, MH, and PH, respectively. The relative values imply that density within PH segments will show modest convergence with other salinity areas as they approach equilibrium but that differences between all other salinity categories will continue to remain stable or diverge slightly.

For the 2002 nesting season, reproductive rates exhibited significant variation among salinity zones (Table 1) and were negatively related to increasing salinity (Fig. 4). Productivity density (reflecting both nesting density and reproductive rates) was -5x higher on average within TF reaches than within PH reaches (Fig. 4A). As with previous parameters, post-hoc comparisons reveal a statistical break along the salinity gradient between OH and MH categories. Per-capita reproductive rates were also negatively associated with salinity (Fig. 4B). However, per-capita rates were more variable, and significant differences were detected only between the extremes (TF vs. PH) of the salinity gradient.

DISCUSSION

Our results show that Bald Eagle nesting densities are associated with the salinity gradient within the lower Chesapeake Bay. Although the association appears to be graded, post-hoc comparisons of population parameters suggest that there is a functional break between low salinity (tidal fresh and oligohaline) and high salinity (mesohaline and polyhaline) waters. This population-level association with salinity has led to significant spatial variation in nesting density. Colonization rate, nesting density, and reproductive rates declined significantly with increasing salt concentrations. Judging from the behavior of the population over the past 26 years, the observed differences in nesting density will probably increase, rather than decrease, as Bald Eagles within the various reaches approach carrying capacity.

Salinity is an abiotic factor that is unlikely to have a direct effect on Bald Eagle nesting distribution. The observed association between nesting parameters and salinity is more likely the result of an indirect effect propagated by an association between salinity and some


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FIG. 4. Mean reproductive rates for Bald Eagles within the study area. Values are presented by salinity zone for (A) 2002 productivity density and (B) 2002 per-capita reproductive rate. Boxes and whiskers represent ?1 SE and ?1 SD about the mean, respectively. Letters provided above values indicate results of post-hoc comparisons. Categories with the same letter designations were not statistically distinguishable.

requirement for nesting. Nesting Bald Eagles typically select territories that (1) are positioned away from human disturbance, (2) provide adequate substrates to support nesting struc- tures, and (3) support enough prey to allow for adequate provisioning of young. Relationships between these factors and salinity may provide further insight into observed nesting patterns and their underlying causes.

In general, Bald Eagles are sensitive to human disturbance and tend to establish nesting territories away from urban locations (e.g. Fraser et al. 1985, Livingston et al. 1990, Therres et al. 1993). Landscapes surrounding Bald Eagle nests in

our study area were significantly different from locations chosen randomly (Watts et al. 1994). Aside from proximity to water, the density of buildings and secondary roads were the habitat dimensions found to be the best predictors of areas used for nesting. Both building and road densities were several times higher around random locations than around nest locations, which suggests that Bald Eagles settle away from human activity. This finding supports an inde- pendent investigation that showed a negative relationship between urban development and the probability that a historical territory (documented prior to the population low) would be recolonized (B. D. Watts unpubl. data).

Although there is clear evidence within the study area that Bald Eagles avoid human activity when selecting nesting territories, there is no indication that human activity and salinity co-vary. When building and road densities are stratified according to salinity zone, there is no significant relationship (Watts et al. 1994). In addition, a recent investigation of the relationship between salinity and impervious surface found no significant relationship (B. D. Watts unpubl. data). Human activity does not appear to be driving the relationship between nesting parameters and salinity within the lower Chesapeake Bay.

Because Bald Eagles build large nests, they require large trees for nesting substrate. And because of their long wingspans, Bald Eagles typically choose large trees where they have direct flight access to the nest position. Under extreme circumstances, lack of available nesting substrate may limit nesting distribution (Hodges 1982). Given their dual requirement for large trees and crown access, the actual relationship between Bald Eagle distribution and forest cover may be influenced by many factors (e.g. topography, landscape structure, forest composition). Landscapes surrounding Bald Eagle nests in our study area were significantly different from locations chosen randomly with respect to forest cover (Watts et al. 1994). On average, the land within 200 m of nests supported four times more mature forest than land surrounding random locations, which suggests that Bald Eagles respond to the distribution of mature forests when selecting territory locations. However, as with human activity, when forest information is stratified according to salinity zone, there is no significant


relationship. Although the exact placement of territories may reflect, in some cases, the distribution of adequate nesting substrate, there is currently no indication that such substrate is driving the observed salinity pattern.

A positive relationship between prey availability and nesting distribution has been documented within several Bald Eagle populations. For example, Dzus and Gerrard (1993) compared prey fish and Bald Eagles from two lakes in north-central Saskatchewan and concluded that prey availability was the most likely cause of differences in nesting density. Swenson et al. (1986) suggested that food availability was the most important factor for selec- tion of nesting areas by Bald Eagles within the Greater Yellowstone Ecosystem. Hansen (1987) experimentally provided food within selected territories along the Chilkat Valley in Alaska and showed that nest success and nestling survival were both higher than at unsupplemented nests serving as controls. Elliott et al. (1998) and Steidl et al. (1997) suggested that prey availability was the most important factor influencing reproductive success for Bald Eagles along the coast of British Columbia and within interior Alaska. These studies point out the influence that food availability has on settlement patterns and productivity.

Several investigators have described the diet of Bald Eagles in the Chesapeake Bay during different seasons (e.g. W. B. Tyrrell unpubl. data 1936, Cline and Clark 1981, Mersmann 1989). Fish are the dominant (>95%) prey resource provided to young (Wallin 1982, Markham 2004). Salinity is one of the principal factors contributing to estuarine fish distribution, and tolerances of species within the Chesapeake Bay are relatively well known (Musick 1972, Murdy et al. 1997). Differences between species have led to predictable distribution patterns and the formation of assemblages that change across the salinity gradient (e.g. Wagner and Austin 1999). However, much less is known about how total fish biomass is related to salinity or, more importantly, how availability of fish to Bald Eagles varies with salinity.

The relationship between salinity and nesting density within the study area is not unique to nesting Bald Eagles. Other piscivorous species and populations exhibit similar distribution patterns. For example, growth rates of the Osprey (Pandion haliaetus) nesting population in the

Chesapeake Bay following dichloro-diphenyl- trichloroethane (DDT)-induced lows are also highest in the lower saline reaches of major tributaries (Watts et al. 2004). The Great Blue Heron (Ardea herodias) nesting population is more abundant in the tidal fresh reaches of the lower Chesapeake Bay (Watts and Byrd 1998). Migrant Bald Eagles from the Southeast use these same waters. The northward migration of Bald Eagles from Florida and other states to the Chesapeake Bay has been known for some time (Broley 1947), but numbers have increased dramatically since the early 1980s (e.g. Watts and Byrd 1999). At least 1,500 Bald Eagles are now moving into bay waters during late spring and summer (B. D. Watts and M. A. Byrd unpubl. data). All six major concentration areas used by these birds through the summer months are within tidal fresh reaches of the bay.

Tidal fresh reaches of the lower Chesapeake Bay appear to be convergence aieas for fish-eat- ing birds, including the Bald Eagle. Both the nesting density and number of young produced per unit area were significantly higher along these shorelines compared with other areas in the lower bay. These areas are also oversummer- ing and molting grounds for birds from nesting populations throughout the Southeast. The conservation implications of these patterns are clear and suggest that land-use decisions within these areas may have far-reaching consequences. However, the specific relationship between the local fisheries and the Bald Eagle population remains unclear. A recent investigation of diet composition within these areas determined that fish within the families Ictaluridae (catfish [Ictalurus spp.]) and Clupeidae (shad and herring [Alosa spp.]) accounted for >70% of the prey items delivered to nestlings (Markham 2004). Catfish are generally considered resident species associated with the less-saline waters of the Chesapeake Bay (Murdy et al. 1997). Prominent members of the Clupeidae are anadromous species that spawn in less-saline waters.

One of the most dramatic events throughout the Chesapeake Bay is the appearance of anadromous fish on spawning runs during the spring months. Four clupeid species-American shad (Alosa sapidissima), hickory shad (A. medi- ocris), alewife (A. pseudoharengus), and blueback herring (A. aestivalis)-spend most of their life cycle in oceanic waters but return to coastal tributaries to spawn (McDowall 1987). Mature


adults enter the bay as water temperatures begin to rise in the spring. Runs to spawning grounds are triggered by temperature thresh- olds required for optimum egg development. During most years, peak spawning occurs during April and May (Klauda et al. 1991a, b). Most Bald Eagle clutches in the lower bay hatch in early April (B. D. Watts and M. A. Byrd unpubl. data), such that the brooding-rearing period coincides directly with these spawning events. All four of these species spawn in upper tributaries where salinity is 0-2 ppt, and most eggs have been collected in areas where salinity is 0-1 ppt. These salinity ranges coincide with the tidal fresh anid upper oligohaline reaches where Bald Eagle nesting densities were comparatively high. After spawning, adults move rapidly downstream to more-saline waters. However, a significant portion of "spent" females die on or near the spawning grounds.

Within the Pacific Northwest, it has been suggested that anadromous fish are keystone species for vertebrate predators and scaven- gers within freshwater streams (Wilson and Halupka 1995). The spectacular annual spawning runs of salmon (Oncorhynchus spp.) are critically important to predators such as the brown bear (Ursus arctos; Shuman 1950, Gard 1971) and Bald Eagle (Hansen 1987, Spencer et al. 1991). The modern-day clupeid runs along the Atlantic Coast are less conspicuous and have received less attention in terms of the ecological link they may provide between aquatic and terrestrial ecosystems. These species spawn within coastal tributaries along most of the Atlantic Coast from the St. Johns River in Florida to Newfoundland, which suggests that their significance to terrestrial species that depend on fish could be much more far-reaching than previously recognized.

ACKNOWLEDGMENTS

Many individuals and organizations have contributed to the success of the Virginia Bald Eagle survey. Several biologists from the Virginia Department of Game and Inland Fisheries have provided oversight and logistical support including, B. Duncan, K. Terwilliger, R. Fernald, D. Bradshaw, D. Schwab, K. Cline, L. Sausville, and J. Cooper. Biologists from the Virginia field office of the U.S. Fish and Wildlife Service, including K.

Mayne, E. Davis, and J. Harrison have also provided information toward the survey. B. Paxton provided expertise with the manipulation of spatial data. Several pilots have provided expert flying services, including S. Beck, C. Shermer, C. Crabbe, and M. Crabbe. Financial support has been provided by the Virginia Department of Game and Inland Fisheries, the U.S. Fish and Wildlife Service, the U.S. Department of Defense, National Park Service, and the U.S. Army Corps of Engineers. We appreciate the many private citizens, bird watchers, and government employees who have reported nest locations over the years.

LITERATURE CITED

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Associate Editor: K. Steenhof

Documents

Salinity and Population Parameters of Bald Eagles (Haliaeetus Leucocephalus) in the Lower Chesapeake Bay