Productivity of Herring Gulls and Lesser Black-backed Gulls in ......Productivity of Herring Gulls Larus argentatus and Lesser Black-backed Gulls L. fuscus in relation to fox predation

Productivity of Herring Gulls Larus argentatus and Lesser Black-backed Gulls L. fuscus in relation to fox predation risk at colonies across northern England

and Wales in 2012

Research Report 61

August 2018

Sarah E Davis1, Linda J Wilson2, Andy Brown3, Leigh Lock4, Elwyn Sharps1 and Mark

Bolton1

1RSPB Centre for Conservation Science, RSPB, The Lodge, Sandy, Bedfordshire, SG19 2DL, UK. 2RSPB Centre for Conservation Science, RSPB, Etive House, Beechwood Park, Inverness, IV2 3BW, UK. 3Natural England, Unex House, Bourges Boulevard, Peterborough, PE1 1NG, UK. 4Nature Recovery Unit, RSPB, The Lodge, Sandy, Bedfordshire, SG19 2DL, UK

This report should be cited as:

Davis, S., Wilson, L. J., Brown, A. and Bolton, M. 2018. Productivity of Herring Gulls Larus argentatus and Lesser Black-backed Gulls L. fuscus in relation to fox predation risk at colonies across northern England and Wales in 2012. RSPB Research Report 61. RSPB Centre for Conservation Science, RSPB, The Lodge, Sandy, Bedfordshire, SG19 2DL.

ISBN: 978-1-905601-57-8

Contents

Summary .................................................................................................................... 4

Introduction ................................................................................................................ 5

Methods ..................................................................................................................... 7

Productivity Estimation ........................................................................................... 7

Monitoring Fox Predation Risk ................................................................................ 8

Effect of Fox Predation Risk on Productivity ........................................................... 9

Effectiveness of Anti-predator Fencing ................................................................... 9

Results ..................................................................................................................... 11

Discussion ................................................................................................................ 12

Acknowledgments .................................................................................................... 16

References ............................................................................................................... 17

Tables ...................................................................................................................... 20

Figures ..................................................................................................................... 28

Summary

The dramatic breeding population declines in Britain and Ireland of Herring Larus argentatus and

Lesser Black-backed Gulls L. fuscus over recent decades have been linked to several causes

including changes in food availability, botulism, predation and culling. This study compared

productivity of both gull species across eight study colonies in northern England and Wales in

relation to the risk of predation from the Red Fox Vulpes vulpes, measured by baited camera traps

and scat transects. In addition, the effects of predator-exclusion fences on nest survival and

productivity were investigated at one of the study colonies. There was a significant negative

relationship between fox sighting rate and productivity, and a significant difference in productivity

(but not nest survival) between fenced and unfenced areas. However, fox sighting rate explained

relatively little variation in productivity, and some colonies with no evidence of fox predation risk

still had relatively low breeding success. For most of the study colonies, breeding success was

below the level required for population stability. The role of fox control measures is discussed

within the context of other potential conservation measures available to help combat population

declines.

Introduction

The population fluctuations of Herring Larus argentatus and Lesser Black-backed Gulls L. fuscus

breeding in the British Isles over the last century have been well-documented (Cramp et al., 1974;

Lloyd et al., 1991; Mitchell et al., 2004; Ross-Smith et al., 2014; Coulson, 2015; Nager and

O'Hanlon, 2016). During the early 20th Century, the populations of both species increased

dramatically but have since declined, with the timing and rate of decline differing between the two

species. Herring Gull numbers have fallen by 60-70% since the early 1970s (Mitchell et al., 2004;

JNCC, 2016) and the species is now a red-listed bird of conservation concern (Eaton et al., 2015).

In contrast, Lesser Black-backed Gulls continued to increase until the turn of the century but

thereafter declined by around 32% between 2000 and 2011 (Balmer et al., 2013). Lesser Black-

backed Gulls are an Amber-listed bird of conservation concern, in part because most of its

population is confined to a few sites (Eaton et al., 2015). The scale of loss from some of these

colonies (e.g. Orford Ness, Suffolk; South Walney, Cumbria; and Bowland Fells, Lancashire)

illustrate the species’ vulnerability (Ross-Smith et al., 2014). The most recent census of the British

and Irish population (at the turn of the 21st Century) estimated a total of 149,177 and 116,684

Apparently Occupied Nests for Herring and Lesser Black-backed Gulls respectively (Mitchell et

al., 2004). However urban areas, where populations are known to have expanded, were not well

covered during the last census or by more recent sample counts, so there is a need for updated

population estimates (Cook and Robinson, 2010; JNCC, 2016). Population trends have varied

between regions during the last four decades (Nager and O'Hanlon, 2016), and though changes

in the numbers of both species have been linked to several possible factors, including changing

availability of food from refuse disposal sites and fisheries waste; botulism; predation and culling

(Mitchell et al., 2004; Coulson, 2015), the causes of change have rarely been quantified.

Here we examine evidence for the impact of predation by Red Foxes Vulpes vulpes. Red foxes

are widespread and numerous in the UK (Webbon et al., 2004), and are known to depredate gull

eggs, chicks and adults (Mavor et al. 2001, Thompson et al. 1998, Campbell 2012), and therefore

capable of affecting both breeding success and adult survival. A meta-analysis of the main factors

contributing to reduced productivity at well-monitored colonies in Britain and Ireland between 1986

and 2006 showed that of 19 cases, only two (Herring Gull) or three (Lesser Black-backed Gull)

reported fox predation as the main driver (Furness et al., 2013). However, in some cases foxes

can have a large impact on gull productivity, for example in 1999, 75% of the c. 30,000 Herring

and Lesser Black-backed Gull nests at Orford Ness in Suffolk failed due to fox predation (Mavor

et al., 2001) and in 2015, a single fox at neighbouring Havergate Island resulted in almost

complete breeding failure for the 3,000 pairs of Herring and Lesser Black-backed Gulls nesting

there (Howe and Record, 2015). Since 1976, both species have increased considerably in

numbers in urban areas, possibly benefitting from the security that urban rooftops can offer from

mammalian ground predators, with breeding success in urban areas tending to be high providing

efforts are not made to deliberately reduce their numbers or productivity (Monaghan and Coulson,

1977; Monaghan, 1979; Raven and Coulson, 1997).

In this study, we investigated whether productivity of Herring and Lesser Black-backed Gulls was

related to the risk of fox predation at eight colonies across England and Wales and assessed the

effectiveness of predator-exclusion fences, a key measure designed to minimise fox predation

(e.g. Malpas et al., 2013). We tested the hypotheses that (i) productivity is lower for both species

where there is higher risk of fox predation and (ii) productivity is higher within areas protected by

predator-exclusion fences than in unfenced areas. By taking a multi-site approach for the first

hypothesis, we aimed to quantify how much variation in productivity could be explained by fox

predation risk to assess whether the effects of this particular pressure were discernible over and

above those caused by other factors thus helping inform the relative benefits of managing fox

predation at gull colonies.

Methods

Eight study colonies were chosen varying in habitat and potential exposure to mammalian ground

predators (Table 1 and 2) and spread across northern England and Wales (though sufficiently

close to allow a team of two people to regularly visit each several times in one season) (Fig. 1).

Six colonies held both Herring and Lesser Black-backed Gulls, while two held only Herring Gulls.

Four of the mixed-species colonies also held small numbers of Great Black-backed Gulls L.

marinus, but the species was not included in the study. At the time of the last seabird census

(1998 – 2002), South Walney and Rockcliffe Marsh were the largest and fourth largest mixed

colonies of Herring and Lesser Black-backed Gulls in Britain and Ireland (Mitchell et al., 2004). In

early 2012 anti-predator electric fences were erected in two of the densest parts of the South

Walney colony, ‘The Spit’ and ‘Gull Meadow’ encompassing an area of approximately 3.6. and

2.4 hectares respectively. This was the only study colony where fox control measures were

employed and they provided an opportunity to investigate the effect of anti-predator fencing.

Productivity estimation

Due to the large differences in size and terrain of the eight colonies, as well as the involvement

of different organisations, methods used to estimate the number of breeding pairs and fledged

chicks differed between colonies (Table 2). For all study colonies (except Rockcliffe Marsh),

counts of Apparently Occupied Nests (AONs) were made using one of the following standard

methods as described in (Walsh et al., 1995): complete counts from suitable vantage points;

complete counts from strip transects; or sample quadrat counts, extrapolated to the area of the

colony, with 95% confidence intervals generated by bootstrapping. The method employed at

Rockcliffe Marsh used average nest density to estimate the total number of AONs across the

entire colony (Campbell, 2012; Coyle, 2012). Given that different methods were used to estimate

AONs among colonies (and sometimes among years within colonies), the number of AONs was

only used to calculate breeding success, and not to compare absolute numbers or population

trends among colonies. As it was not feasible to routinely distinguish between the nests of each

species, the proportion of the total AONs that related to each species was based on the observed

ratio of adults within a series of sample counts at each colony. Productivity for each species was

estimated using either observations of a sample of marked nests from vantage points, or mark-

recapture of chicks identified to species, following (Walsh et al., 1995) (Table 2). Chick mark-

recapture was undertaken across the whole of each colony except for South Walney (see below)

and for Langden Head and Ribble Estuary, where the large number of chicks meant that a more

manageable sub-area of the colony had to be used. Breeding success at each colony was

calculated as the number of fledglings divided by the number of AONs.

Monitoring Fox Predation Risk

Logistical and resource constraints precluded the monitoring of fox predation across the study

colonies, so an indication of fox predation risk was provided by fox activity levels, measured using

a combination of scat transects and baited camera traps at each colony (except Puffin Island,

which was known to be free of mammalian predators, (Stanbury et al., 2017)). Scat transects (3.0

– 6.3km in length, see Table 3) were undertaken once a month from May to July (three in total

per colony). As far as possible, these followed linear features such as fence lines, tracks, paths

and edges of creeks on salt marsh areas, as foxes tend to follow such features. Any scats, as

well as other evidence of predatory mammals, such as tracks or prey remains, was recorded and

removed to prevent double counting during subsequent transect surveys. Although none of the

transects at South Walney crossed into the fenced areas, some sections ran alongside the fence

line where observers checked both sides of the fence line (the entirety of the fence at The Spit

(938m) and the southern edge of the fence at Gull Meadow (595m)).

Between two and four baited camera traps were set up at points in or around each colony near

linear features, often along the scat transect route, giving a second but non-independent measure

of fox activity (Table 3). At South Walney, one camera was positioned within each of the four

areas (two fenced, two unfenced). Cameras were set up between 20 April – 8 May 2012 and

deployed until 11-28 July 2012. A small amount of dog food (Chappie brand) was buried in a

shallow hole and a Bushnell trail camera ‘Trophy Cam’ set up to take photos of anything that

visited the food. Cameras were secured to posts approximately one metre from ground level and

two metres from the bait, angled to ensure the bait was in line of sight. The infra-red motion

activated camera was set to take two photos every time it was triggered, on ‘normal’ sensitivity,

with an interval of two seconds before it could be triggered again. Burying the food in a shallow

hole was intended to prevent other animals and birds taking the food, and to reduce the smell

sufficiently that it would only attract predators that passed within a few metres of the food (Howell,

2011). The bait was checked after approximately ten days, after which it was noted whether the

bait had gone (‘depredated’) and the camera was moved to a new location and re-baited to avoid

predators learning the location of the bait. The photos taken by the camera were then checked to

identify the species responsible for any food removed, and an average nightly sighting rate across

the camera trials for each colony calculated for each predator species to give an index of predator

activity levels as an indication of predation risk. The scat transects and cameras at St Bees Head

and Bempton Cliffs were situated along the clifftop rather than in the colony itself, and although

many mammalian ground predators have the ability to traverse cliff ledges, some of the gull nest

locations may have been inaccessible to them.

Only the sightings data for foxes were sufficient to include in the analysis. Both scat transects and

baited camera traps suffer from potential biases when used to measure activity levels therefore

we also scored each colony for fox presence/absence based on whether one or both methods

recorded fox activity (present) or if neither recorded fox activity (presumed absent).

Effect of Fox Predation Risk on Productivity

To assess whether there was an effect of fox predation risk on gull productivity, we used

generalized linear mixed models (GLMMs) with binomial error and logit link function. Models were

fitted in the ‘lme4’ R package (Bates et al., 2015). The response variable was the number of

fledged chicks (measured directly as described above) and failed chicks (estimated from an

assumed mean clutch size of 2.6 for both species based on (Cramp and Simmons, 1983) and the

number of breeding pairs), thus accounting for varying numbers of nests surveyed, and

representing productivity as ‘fledging probability per egg’ (Cook et al., 2014; Carroll et al., 2015;

Carroll et al., 2017). Each measure of fox predation risk (scat density, nightly sighting rate and

fox presence/absence) was fitted in separate models as fixed factors. Gull species was included

as a fixed factor and colony as a random factor. An observation-level random effect was included

to account for over-dispersion (Browne et al. 2005), and model diagnosis carried out using the R

package ‘DHARMa’ (v0.1.5) (Hartig, 2017). Model performance was assessed using AIC: AICs

were compared with that from a null model, fitted with intercept and random effect only: the model

with lowest AIC was deemed the best performing; those within 2 AIC units of the best were

considered to show equivalent support (Burnham and Anderson, 2002).

Effectiveness of Anti-predator Fencing

Samples of nest histories were recorded within and outside of the fenced areas at both Gull

Meadow and The Spit (total of 108 (Herring Gull) and 97 (Lesser Black-backed Gull) nests, Table

4). Each nest was marked with a small cane and visited at least once per week to record the

number of eggs present until they had either hatched or the nest had failed. Nest survival (in this

case, the probability that a nest survives from initiation to hatching at least one chick) was

examined in relation to plot (Gull Meadow or The Spit), treatment (fenced or unfenced) and

species, using the package “RMark” in the statistical programme “R” (Laake and Rexstad, 2008).

RMark uses numerical maximum-likelihood techniques and computes a quasi-likelihood AIC

value (White and Burnham, 1999). This enabled the selection of the variable(s) that most strongly

accounted for variation in daily nest survival. This method does not assume that nest visits are

made at regular intervals (Johnson, 2007) and is computed using an encounter history for each

nest based on: the first day the nest was found, the last day the nest was known to be active, the

last day that the nest was checked, and the fate of each nest (success or fail). Incubation period

ranges from 28-30 days (Herring Gulls) and 24-27 days (Lesser Black-backed Gulls) (Cramp and

Simmons, 1983). Therefore, to estimate nest survival over this period, the daily survival rate was

raised to the power of either 29 days (Herring gull) or 25.5 days (Lesser Black-backed Gull)

following (Rotella et al., 2009). Variance was calculated using the Delta method (Powell, 2007)

from which confidence intervals were calculated, following (Armstrong et al., 2002). Model outputs

were compared using AIC corrected for small sample size (Burnham and Anderson, 2002). In

addition, we compared the number of fledged and failed chicks between fenced and unfenced

areas for The Spit plot using a Chi squared test for each species. The extensive nature of the

colony and uniform terrain prevented assessment of the number of fledged chicks in the unfenced

Gull Meadow area at South Walney; this was not an issue at the unfenced area at The Spit, which

acted as a natural boundary and restricted chick movement. All analyses were conducted in R

v.3.4.1 (R Core Team, 2017).

Results

Study colonies varied in observed breeding success, ranging from complete breeding failure for

both species at Hodbarrow, which was also the smallest colony for both species, up to a breeding

success of 2.22 and 1.81 fledglings per pair for Herring Gull and Lesser Black-backed Gull

respectively at Langden Head (Table 2). The best supported model showed there was a

significant negative effect of fox sighting rate, with productivity decreasing with increasing fox

sighting rate (Table 5 and 6, Fig. 2), supporting our first hypothesis. An equally supported model

also included an effect of species, with Lesser Black-backed Gulls having consistently lower

productivity compared to Herring Gull, but in this model neither the effect of fox sighting rate or

species was significant at P=0.05 (Table 5 and 6). The conditional R2 (the proportion of variance

explained by both the fixed and random variables) for the top two ranking models was 0.67 in

both cases, compared to a conditional R2 of 0.53 for the null model containing the same random

structure but no fixed variables. None of the other models containing different measures of fox

predation risk performed any better than the null model. We used the top ranking model to make

predictions, as this was the most parsimonious model. This showed that when fox sighting rate is

zero, the average probability of an egg fledging a chick across colonies/species is predicted to be

0.31± 0.17 and is reduced to effectively zero (

Discussion

We found that productivity of both Herring and Lesser Black-backed Gulls across our eight study

colonies reduced as fox sighting rate increased and that this effect was significant. Evidence of

fox predation was not systematically collected as part of this study, so we are unable to confirm

whether fox sighting rates were correlated with levels of fox predation. However at Hodbarrow,

which was the colony with the highest average fox sighting rate (0.19 foxes per night), all eggs

were depredated and foxes were thought responsible for the resultant zero productivity

(Blackledge et al., 2013). In comparison at Rockcliffe Marsh, where average fox sighting rate was

much lower (0.04 foxes per night), only two dead chicks were found (cause of death unknown)

and fox predation was deemed to be the most likely cause of death for only six of the 34 adult

gulls found dead (Campbell, 2012). The main contributory factors to poor productivity at Rockcliffe

Marsh were thought to be due to poor food availability, flooding by tidal inundation, adverse

weather, trampling of nests by livestock and intra-specific predation rather than fox predation

(Campbell, 2012).

Although no foxes were recorded by our cameras at South Walney, foxes were observed on site

- including within both fenced areas - and large numbers of depredated chicks were found (S.

Davis, pers. obs). Solar-panel charged batteries struggled to maintain high voltage in the fences,

and parts of the fence were also damaged by livestock (S. Davis, pers. obs), thus compromising

the integrity of the fences. Foxes were thought to be responsible both for the large numbers of

depredated chicks found in 2012 as well as for the poor productivity / breeding failure for a number

of years preceding our study (Norris and Raven, 2011). For South Walney therefore, fox sighting

rate did not appear to reflect fox predation risk.

Despite breaching of both fenced plots at South Walney by foxes, the predator-exclusion fencing

appeared to have some beneficial effect, with productivity being significantly higher than expected

within the fenced areas compared to the unfenced areas at The Spit. This appeared to be largely

due to higher survival at the chick stage, as there was little evidence to show any effect of fencing

on nest survival rates, which were high both within and outside the fenced area. If fox incursions

tend to be more prevalent once chicks have hatched, then this will be of relevance when

considering the optimal timing of any lethal control measures. The fact that breeding success in

the fenced area at Gull Meadow was relatively low (0.15 and 0.25 fledglings/pair for Herring Gull

and Lesser Black-backed Gull respectively) and, for Lesser Black-backed Gull, comparable to the

unfenced rather than the fenced area at The Spit (0.26 fledglings/pair) is perhaps surprising. It

was not possible to collect productivity data for the unfenced area at Gull Meadow so we were

unable to assess the effect of fencing on productivity for this plot. However, a pilot study in 2010

compared the number of chicks fledged per ‘active nest’ (a nest with at least one egg) within and

outside of a 25x75m predator-exclusion fenced area within Gull Meadow. This recorded 0.79

fledglings per active nest (n=77) inside the fence compared to 0.15 fledglings per active nest

(n=77) outside the fence (nests and chicks were not identified to species) (Norris and Raven,

2011). The study was repeated in 2011 but the fencing was breached by foxes and no chicks

survived to fledging in either plot (Norris and Raven, 2011). Based on these previous findings,

the fact that breeding success in 2012 within the fenced area at Gull Meadow was comparable to

the unfenced area at The Spit suggests that the fox(es) that breached the Gull Meadow fenced

area in 2012 may have had a greater impact on productivity compared to The Spit fenced area.

This might occur if the fence was breached at Gull Meadow when the chicks were younger and

therefore more vulnerable to being depredated, and indeed fox incursions into The Spit area

generally tend to occur later in the season, when the chicks are more mobile (Dalrymple, 2017).

Although we found a significant negative relationship between fox sighting rate and productivity,

fox sighting rate explained relatively little additional variation (14%) in productivity compared to

the null model. The significant negative relationship was probably driven by Hodbarrow, which

had the highest fox sighting rate and was the only study colony to suffer complete breeding failure.

Indeed, when the Hodbarrow data are removed from the analysis, the effect is no longer

significant and the most plausible model is the null model (results not shown). The fact that some

colonies with no evidence of fox predation risk still had relatively low productivity (e.g. Puffin

Island) coupled with the low amount of variation in productivity explained by fox sighting rate

supports previous studies that indicate there is no single cause for the observed population

changes in these large gull species (Nager and O'Hanlon, 2016). Based on the colonies sampled

in our study in 2012, it appears that fox predation was not in fact a major widespread problem but

more of a localised issue at two colonies in particular, South Walney and Hodbarrow.

The estimated breeding success required for population stability for Herring Gulls is 1.3-1.5

chicks/pair (Cook and Robinson, 2010). The observed Herring Gull breeding success in 2012

exceeded this level at only two of our study colonies: Langden Head (where there was no

evidence of foxes) and St Bees Head (which had the lowest fox sighting rates of the four colonies

where foxes were sighted). Although limited breeding success data have been collected at our

study colonies in previous years or since, the values we recorded in 2012 are similar to the

average and range of values recorded during other years between 2008 and 2017, for years and

colonies where those data are available (JNCC, 2018), indicating that 2012 represented a typical

year.

Even at fox sighting rates of zero, the modelled probability of an egg fledging a chick is low (0.31±

0.17 chicks fledged/egg) and equivalent to 0.81 chicks fledged/pair (based on a mean clutch size

of 2.6). This suggests that even if fox control measures were to be implemented at those sites

where they are present, any resulting increases in productivity may still fall far short of what is

needed to maintain the population. However, this is based on our ‘average’ study colony and the

degree of positive effects of fox control measures are likely to be highly site specific. The fenced

plots have been maintained at South Walney (with additional various improvements to prevent

fox incursions) and population monitoring indicates that numbers have stabilised since the fences

have been in place, with productivity now being affected by corvid predation and food availability

rather than predation from ground predators (Dalrymple, 2017). Floating fences were installed at

Hodbarrow in 2013 and have subsequently been extended and improved. Monitoring using scat

transects and thermal imaging cameras shows the fences at Hodbarrow to be successful in

keeping foxes out, but large gulls are now discouraged from settling in an effort to protect breeding

Little Terns Sternula albifrons (Blackledge and Maclauchlan, 2017).

Potential conservation measures for increasing survival / breeding success of Herring and Lesser

Black-backed Gulls alongside fox control include: (i) a cessation of culling of gulls; (ii) the

management of other mammalian predators at breeding sites, particularly the eradication of non-

native Brown Rat Rattus norvegicus and American Mink Neovison vison; iii) habitat management

to reduce risk of flooding and increase availability of nesting habitat (typically short vegetation that

provides cover for nests and chicks) and, for Lesser Black-backed Gull which are more dependent

on small pelagic fish than Herring Gull, (iv) the closure of sandeel and sprat fisheries close to

breeding colonies (Furness et al., 2013; Ross-Smith et al., 2015). An additional consideration is

the impact that both gull species have on other species of conservation concern through predation

or displacement (e.g. nesting terns at Hodbarrow), an issue further exacerbated by a lack of

suitable breeding habitat, forcing their co-existence within increasingly smaller areas. Therefore,

while fox control measures are likely to have a positive effect on productivity levels, improve

resilience of the population to other pressures, help to combat population declines, and may be

the most important action for some sites, they should be viewed as only one component of a suite

of measures that are likely to be required to aid population resilience and recovery in these two

gull species. The most appropriate conservation management strategy will therefore need to

apply the most relevant measures to each site.

Acknowledgments

This work was funded by Natural England and RSPB through the NE/RSPB Action for Birds in

England partnership. Permission to access the colonies was kindly provided by Cumbria Wildlife

Trust (Rockcliffe Marsh, South Walney), United Utilities (Langden Head), Sir Richard Williams

Bulkeley and Countryside Council for Wales (now Natural Resources Wales) (Puffin Island),

Natural England (Ribble Estuary) and RSPB (St Bees Head, Hodbarrow and Bempton Cliffs). We

would like to thank all of the site staff and volunteers at all of the colonies we worked at, whose

help with this work was invaluable. Particular thanks go to the following people: Liz Morgan, Andy

Schofield, Becca Oswain, Katie Fuller, Steve Wake, Lee Schofield, Mike Carrier, Jenny Campbell,

Norman Holton, Dave Blackledge, Mhairi Maclauchlan, Peter Jones, Adam Maher, plus

volunteers, Peter Wilson, Jude Lane, Mick Demain, Shaun Coyle, Dave Mercer, David Aitken,

Steve Dodd, Liverpool University, Johann Bourgeois, Jonathan Green, Louise Soanes, Zac

Hinchliffe, Amanda Kuepfer, Robin Sandham. We thank Daisy Burnell (JNCC) for kindly providing

productivity data from JNCC’s Seabird Monitoring Programme, and Alex Banks and Tim Frayling

(Natural England) for providing helpful comments on an earlier draft of this paper.

References

Armstrong, D. P., Raeburn, E. H., Powlesland, R. G., Howard, M., Christensen, B., and Ewen, J. G. 2002. Obtaining meaningful comparisons of nest success: data from New Zealand robin (Petroica australis) populations. New Zealand Journal of Ecology, 1: 1-13.

Balmer, D. E., Gillings, S., Caffrey, B. J., Swann, R. L., Downie, I. S., and Fuller, R. J. 2013. Bird Atlas 2007-11: the breeding and wintering birds of Britain and Ireland. BTO Books, Thetford.

Bates, D., Mächler, M., Bolker, B., and Walker, S. 2015. Fitting Linear Mixed-Effects Models Usinglme4. Journal of Statistical Software, 67.

Blackledge, D., Holton, N., and Maclauchlan, M. 2013. Hodbarrow Reserve Annual Report for 2012/13. Unpublished RSPB report.

Blackledge, D., and Maclauchlan, M. 2017. Hodbarrow Reserve Annual Report for 2016/17. Unpublished RSPB report.

Burnham, A., and Anderson, D. R. 2002. Model selection and multi model inference: A practical information theoretic approach (2nd ed.). New York, NY: Springer Verlag.

Campbell, J. 2012. Rockcliffe Marsh Wardens Report 2012. Unpublished report, Cumbria Wildlife Trust.

Carroll, M. J., Bolton, M., Owen, E., Anderson, G. Q. A., Mackley, E. K., Dunn, E. K., and Furness, R. W. 2017. Kittiwake breeding success in the southern North Sea correlates with prior sandeel fishing mortality. Aquatic Conservation: Marine and Freshwater Ecosystems, 0: 1-12.

Carroll, M. J., Butler, A., Owen, E., Ewing, S. R., Cole, T., Green, J. A., Soanes, L. M., et al. 2015. Effects of sea temperature and stratification changes on seabird breeding success. Climate Research, 66: 75-89.

Cook, A. S. C. P., Dadam, D., Mitchell, I., Ross-Smith, V. H., and Robinson, R. A. 2014. Indicators of seabird reproductive performance demonstrate the impact of commercial fisheries on seabird populations in the North Sea. Ecological Indicators, 38: 1-11.

Cook, A. S. C. P., and Robinson, R. A. 2010. How Representative is the Current Monitoring of Breeding Seabirds in the UK? BTO Research Report No. 573.

Coulson, J. C. 2015. Re-Evaluation of the Role of Landfills and Culling in the Historic Changes in the Herring Gull (Larus argentatus) Population in Great Britain. Waterbirds, 38: 339-354.

Coyle, P. 2012. The Origins and Development of the Langden Head Gullery. Unpublished Natural England Report.

Cramp, S., Bourne, W. R. P., and Saunders, D. 1974. The seabirds of Britain and Ireland. Collins, London, U.K.

Cramp, S., and Simmons, K. E. L. 1983. The birds of the Western Paleartic. Vol. III, Oxford University Press, Oxford.

Dalrymple, S. 2017. Report on the Herring and Lesser Black-backed Gull Colony at South Walney Nature Reserve 2017. Unpublished Cumbria Wildlife Trust report.

Eaton, M., Aebischer, N., Brown, A., Hearn, R., Lock, L., Musgrove, A., Noble, D., et al. 2015. Birds of Conservation Concern 4: the population status of birds in the UK, Channel Islands and Isle of Man. British Birds, 108: 708-746.

Furness, B., MacArthur, D., Trinder, M., and MacArthur, K. 2013. Evidence review to support the identification of potential conservation measures for selected species of seabirds. Report to Defra under contract MB0138.

Hartig, F. 2017. DHARMa: Residual Diagnostics for hierarchical (Multi-Level / Mixed) Regression Models. R package version 0.1.5. http://florianhartig.github.io/DHARMa/.

http://florianhartig.github.io/DHARMa/

Howe, A., and Record, L. 2015. Havergate Reserves South Suffolk Coast. Annual Report for 2014/15.

Howell, N. 2011. Distribution of Red Fox (Vulpes vulpes) and its effect on predation of Lapwing (Vanellus vanellus) nests of a lowland wet grassland. Unpublished MSc thesis. University of East Anglia, Norwich.

JNCC 2016. Seabird Population Trends and Causes of Change: 1986-2015 Report (http://jncc.defra.gov.uk/page-3201). Joint Nature Conservation Committee. Updated September 2016.

JNCC 2018. Seabird Monitoring Programme database http://jncc.defra.gov.uk/smp accessed 26 Feb 2018.

Johnson, D. H. 2007. Estimating nest success: a guide to the methods. Studies in Avian Biology, 34: 65.

Laake, J., and Rexstad, E. A. 2008. RMark–an alternative approach to building linear models in MARK. Pages C1–C115 in EG Cooch and GC White (Eds). Program MARK: a gentle introduction.

Lloyd, C. S., Tasker, M. L., and Partridge, K. 1991. The Status of Seabirds in Britain and Ireland. , T & AD Poyser, Calton.

Malpas, L. R., Kennerley, R. J., Hirons, G. J., Sheldon, R. D., Ausden, M., Gilbert, J. C., & Smart, J. 2013. The use of predator-exclusion fencing as a management tool improves the breeding success of waders on lowland wet grassland. Journal for nature conservation, 21(1), 37-47.

Mavor, R. A., Pickerell, G., Heubeck, M., and Thomson, K. R. 2001. Seabird numbers and breeding success in Britain and Ireland, 2001. Joint Nature Conservation Committee, Peterborough, UK.

Mitchell, P. I., Newton, S. F., Ratcliffe, N., and Dunn, T. E. 2004. Seabird populations of Britain and Ireland, T. & A. D. Poyser, London.

Monaghan, P. 1979. Aspects of the breeding biology of Herring Gulls Larus argentatus in urban colonies. Ibis, 121: 475-481.

Monaghan, P., and Coulson, J. C. 1977. The status of large gulls nesting on buildings. Bird Study, 24: 89-104.

Nager, R. G., and O'Hanlon, N. J. 2016. Changing Numbers of Three Gull Species in the British Isles. Waterbirds, 39: 15-28.

Norris, M., and Raven, C. 2011. A study into productivity levels amongst the mixed ground nesting gull colony at South Walney. Unpublished Cumbria Wildlife Trust report. .

Powell, L. A. 2007. Approximating variance of demographic parameters using the delta method: a reference for avian biologists. The Condor, 109: 949-954.

R Core Team 2017. R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. URL https://www.R-project.org/.

Raven, S. J., and Coulson, J. C. 1997. The distribution and abundance of Larus gulls nesting on buildings in Britain and Ireland. Bird Study, 44: 13-34.

Ross-Smith, V. H., Grantham, M. J., Robinson, R. A., and Clark, J. A. 2014. Analysis of Lesser Black-backed Gull data to inform meta-population studies. BTO Research Report No. 654.

Ross-Smith, V. H., Johnston, A., and Ferns, P. N. 2015. Hatching success in Lesser Black-backed Gulls Larus fuscus - an island case study of the effects of egg and nest site quality. Seabird, 28: 1-16.

Rotella, J., Cooch, E., and White, G. 2009. Nest survival models. Program Mark: a gentle introduction: 17.

Stanbury, A., Thomas, S., Aegerter, J., Brown, A., Bullock, D., Eaton, M., Lock, L., et al. 2017. Prioritising islands in the United Kingdom and crown dependencies for the eradication of

http://jncc.defra.gov.uk/page-3201http://jncc.defra.gov.uk/smphttps://www.r-project.org/

invasive alien vertebrates and rodent biosecurity. European Journal of Wildlife Research, 63.

Walsh, P. M., Halley, D. J., Harris, M. P., del Nevo, A., Sim, I. M. W., and Tasker, M. L. 1995. Seabird Monitoring Handbook for Britain and Ireland. JNCC/RSPB/ITE/Seabird Group, Peterborough.

Webbon, C. C., J., B. P., and Harris, S. 2004. Faecal density counts for monitoring changes in red fox numbers in rural Britain. Journal of Applied Ecology, 41: 768-779.

White, G. C., and Burnham, K. P. 1999. Program MARK: survival estimation from populations of marked animals. Bird Study, 46: S120-S139.

Tables

Table 1. Characteristics of the eight study colonies. HG = Herring Gull; LBBG = Lesser Black-

backed Gull; GBBG = Great Black-backed Gull.

Colony Species Site description

Rockcliffe Marsh

HG

LBBG

GBBG

Coastal saltmarsh, grazed by cattle and sheep.

Subject to flooding during spring tides early in the

season

St Bees Head

HG Coastal cliff at the edge of farmland

Hodbarrow

HG

LBBG

Very small shingle islands in a sheltered coastal

lagoon, approx. 3 metres from mainland at

narrowest point

South Walney

HG

LBBG

GBBG

Dune and shingle area of Walney ‘Island’ which is

connected to the mainland by bridge at north end.

Langden Head

HG

LBBG

Inland colony on moorland in the forest of

Bowland. Surrounded by shooting estates with

high levels of keepering.

Ribble Estuary

HG

LBBG

GBBG

Coastal saltmarsh, grazed by cattle and sheep.

Subject to flooding during spring tides early in the

season

Bempton Cliffs

HG Coastal cliff at the edge of farmland/nature reserve

Puffin Island

HG

LBBG

GBBG

Coastal island

Table 2. Colony counts and breeding success for each species. Sample sizes (AONs) for

breeding success estimates are provided in parenthesis where these were not based on whole

colony counts. For South Walney, proportions of each species were counted at the level of the

quadrat, thus allowing 95% confidence intervals to be generated.

Herring Gull Lesser Black-

backed Gull

Count

method

Productivit

y method

AONs Breeding

success

(fledglings

per pair)

AONs Breeding

success

(fledgling

s per pair)

Bempton

Cliffs

n/a Vantage

point counts

Not

counted

0.92 (n=12)

0 -

Rockcliffe

Marsh

Nest density Mark

recapture

560 0.01

2740 0.02

St Bees Head Vantage point

counts

Vantage

point counts

195 1.53 (n=29)

0 -

Hodbarrow Vantage point

counts

Vantage

point counts

5 0.00

60 0.00

Walney

(whole

colony)

Sample

quadrats

(n=35)

n/a 1636

(95%

CIs:

1408-

1864)

Not

monitored

4699

(95%

CIs:

4050-

5347)

Not

monitored

Walney

(Fenced, The

Spit)

Transects Mark

recapture

427 1.18 589 0.68

Walney

(Unfenced,

The Spit)

Transects Mark

recapture

842 0.40

1266 0.26

Walney

(Fenced, Gull

Meadow)

Transects Mark

recapture

85 0.15

240 0.25

Walney

(Unfenced,

Gull Meadow)

Transects n/a Not

counted

Not

monitored

Not

counted

Not

monitored

Langden

Head

Sample

quadrats

(n=12)

Mark

recapture

60 2.22 (n=9)

3957 1.81

(n=616)

Ribble

Estuary

Sample

quadrats

(n=36)

Mark

recapture

1346 0.93

(n=204)

8267 0.05

(n=1250)

Puffin Island Transects Mark

recapture

492 0.61

579 0.50

Table 3. Information on predators at each colony collected from scat transects and baited camera traps. GM = Gull Meadow.

Scat transects Baited camera traps Red Fox

presence/

absence

Colony Length

of scat

transect

Average no.

fox

scats/km/day

No. of

camera

trials

(no. of

cameras)

Total

no.

camera

nights

Average nightly sighting rate

Red Fox Rat

Rattus

sp.

Badger

Meles

meles

Mustelidae European

Hedgehog

Erinaceus

europaeus

Bempton

Cliffs

3.0 km 0.333 11 (2) 106 0.05 0.01 0 0 0 1

Rockcliffe

Marsh

4.8 km 0 24 (3) 220 0.04 0 0 0 0 1

St Bees

Head

3.5 km 0.095 17 (2) 159 0.03 0.02 0 0 0.06 1

Hodbarrow 4.7 km 0.284 16 (2) 152 0.19 0 0.01 0 0.01 1

South

Walney:

GM,

fenced

0.59km 0 12 (1) 67 0 0.15 0 0 0 0

Spit,

fenced

0.94km 0 16 (1) 73 0 0 0 0 0 0

GM,

unfenced 6.3km 0.053

12 (1) 75 0 0.03 0 0 0 1

Spit,

unfenced

17 (1) 57 0 0 0 0 0 1

Langden

Head

4.4 km 0 20 (3) 267 0 0 0 0.01 0.01 0

Ribble

Estuary

5.0 km 0 29 (3) 282 0 0 0 0 0 0

Puffin

Island

None Assumed

zero

n/a n/a Assumed

zero

n/a n/a n/a n/a 0

Table 4. Summary of the data used in the nest survival model and causes of nest failure.

Species Fenced/

Unfenced

Plot Total

nests

No.

successful

No.

failed

Causes of failure

No.

predated

No. trampled No. deserted No. missing

Herring

Gull

Fenced Gull

Meadow

13 10 (77%) 3

(23%)

2 1

The Spit 36 34 (94%) 2

(6%)

2

Unfenced Gull

Meadow

14 11 (79%) 3

(21%)

3

The Spit 45 41 (91%) 4

(9%)

1

3

Lesser

Black-

backed

Gull

Fenced Gull

Meadow

34 29 (85%) 5

(15%)

2 2

1

The Spit 13 12 (92%) 1

(8/%)

1

Unfenced Gull

Meadow

38 36 (95%) 2

(5%)

2

The Spit 12 12 (100%) 0

(0%)

27

Table 6. Parameter estimates for the top ranked models in Table 5.

Model AIC Parameter Estimate ± SE P value

AIC= 204.2 Fox_sighting_rate -41.43 ± 20.28 0.041

AIC=205.7 Fox_sighting_rate -41.62 ± 21.83 0.057

SpeciesLBBG -0.88 ± 0.55 0.111

Table 7. Results of the RMark nest survival analysis. Delta AICc: the difference between the

model in question, and the top model. Akaike weights represent the relative likelihood of the

models (i.e. exp (-0.5 *Delta AICc) for each model, divided by the sum of these values across

all other models).

Model AICc Delta AICc Weight

Null model (constant survival) 1098.252 0 0.334206

Species 1100.064 1.811832 0.135076

Plot 1100.122 1.869632 0.131229

Treatment 1100.195 1.942732 0.126519

Species + Plot 1101.618 3.366131 0.062097

Species + Treatment 1101.999 3.747031 0.051328

Treatment + Plot 1102.071 3.818631 0.049523

Species + Treatment + Plot 1103.558 5.306197 0.023539

Species * Plot 1103.568 5.316297 0.02342

Treatment * Plot 1103.716 5.463797 0.021755

Species * Treatment 1103.762 5.509597 0.021263

Species + Treatment * Plot 1105.249 6.996831 0.010108

Species * Treatment + Plot 1105.283 7.031031 0.009937

28

Figures

Figure 1. Map of northern England and Wales showing the locations of the eight study

colonies.

Figure 2. Relationship from the top ranking model of gull productivity (fledging probability

per egg) in relation to fox sighting rate. Solid line indicates fitted relationship, with dashed

lines indicating ±1 standard error. Open circles indicate data points (number of fledglings per

egg, based on raw data and a mean clutch size of 2.6).

Documents

Productivity of Herring Gulls and Lesser Black-backed Gulls in ......Productivity of Herring Gulls Larus argentatus and Lesser Black-backed Gulls L. fuscus in relation to fox predation