productivity of herring gulls and lesser black-backed gulls in ......productivity of herring gulls...
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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.
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ISBN: 978-1-905601-57-8
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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
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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.
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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
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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.
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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-
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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
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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
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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).
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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 (
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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
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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
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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
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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.
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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.
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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
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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
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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
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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%)
-
Table 5. Model selection table for analysis of the effect of fox predation risk on productivity, with the top ranked models highlighted in
bold. Delta AICc: the difference between the model in question, and the top model.
Model AICc Delta AICc
Fox_sighting_rate+(1|Colony) +(1|Obs) 204.2 0
Fox_sighting_rate+Species+(1|Colony) +(1|Obs) 205.7 1.49
1+(1|Colony) +(1|Obs) 207.2 2.94
Species+(1|Colony)+(1|Obs) 208.2 4.03
Fox_presence+(1|Colony) +(1|Obs) 209.6 5.34
Fox_scat_density+(1|Colony)+(1|Obs) 209.9 5.73
Fox_sighting_rate*Species+(1|Colony) +(1|Obs) 210.3 6.12
Fox_presence+Species+(1|Colony) +(1|Obs) 211.0 6.78
Fox_scat_density+Species+(1|Colony)+(1|Obs) 211.4 7.19
Fox_scat_density*Species+(1|Colony)+(1|Obs) 214.4 10.15
Fox_presence*Species+(1|Colony) +(1|Obs) 215.5 11.25
-
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).