Running head: Managing gull predation

An evaluation of canes as management technique to reduce predation by gulls on ground nesting seabirds

CLAIRE BOOTHBY1, CHRIS REDFERN2 & JULIA SCHROEDER1*

1 Department of Life Sciences, Silwood Park Campus, Imperial College London, SL5 7PY Ascot, UK

2School of Biology, Newcastle University, NE1 7RU Newcastle upon Tyne, UK

*Corresponding Author

Email: julia.schroeder@gmail.com

The best documented method to decrease predation on breeding seabirds by gulls are culling regimes; however, studies on the effectiveness of this method show ambiguous results. Here we tested an alternative method, using bamboo canes erected in four breeding sites of Arctic Tern Sterna paradisaea as gull deterrent. While we found fewer predation attempts in the caned areas than in the control areas, there was no effect of canes reducing the probability of predation success per attempt. This pilot study documents that the use of canes as a conservation strategy to reduce gull predation is promising, economic and simple, and thus likely a versatile tool for conservation managers.

Key-words: conservation, deterrent, predator-prey dynamics, Sterna paradisaea, wildlife management

Approximately one third of seabird species are threatened or critically endangered (IUCN 2012). Population declines have been attributed to direct and indirect impacts of fisheries, pollution, habitat degradation and alien invasive species (Croxall et al. 2012). Whilst not considered to be a major cause for the seabird decline, on the local scale predation by native predatory gulls can limit the productivity of seabirds, including terns Sternidae (Whittam & Leonard, 1999, Hernández-Matías et al. 2003, Ackerman et al. 2014), petrels Procellariiformes (Libois et al. 2012), Guillemots Uria aalge (Birkhead 1977) and skimmers Rynchopidae (O’Connell & Beck 2002). Predation has also led to the abandonment of specific breeding grounds (Whittam & Leonard, 1999).

It is thought that predation pressure by gulls on other seabirds has risen because gull numbers increased over the early half of the last century (Ackerman et al. 2014, Coulson 2015). Yet, culling gulls as a conservation technique is controversial (Harris & Wanless 1997, Bosch 2000, Finney et al. 2003, Sanz-Aguilar et al. 2009). Also, reports of gull declines in the last half of the 20th century (Mitchell et al. 2004, Ross-Smith et al. 2014, Coulson 2015) require conservation managers to balance the needs for protection of one species in light of those needs of all species.

One alternative to culling is the translocation of a gull colony, which increased tern chick survival by 900% (Ackerman et al. 2014), but this is expensive, requires the availability of suitable habitat and still leads to local decline in gulls. Simpler and cheaper measures might be more useful for conservation managers. One study on a colony of Black Skimmer Rynchaps niger found no effect of deploying effigies (Forys et al. 2015). Blokpoel et al. (1997) found some success with monofilament lines, which prevent gulls landing and nesting, while others have found this method largely ineffective (Morris et al.1992, Scopel & Diamond 2017).

Another promising strategy is erecting canes within the colonies of ground nesting seabirds, to deter predation by gulls. The canes could also slow predators down and therefore reduce the amount of successful predation attempts. The effectiveness of this strategy has, to our knowledge, not yet been quantified.

The aim of this study was to quantify experimentally the effectiveness of canes as a conservation tool to limit predation by large gulls. We test two hypothesises: 1) Canes reduce the number of predation attempts from large gulls. 2) Predation attempts in caned areas are less likely to be successful than in areas without canes.

METHODS

The fieldwork for the experiment was conducted by CB on the Inner Farne and Brownsman Islands, part of an archipelago known as the Farne Islands, located off the North East coast of the United Kingdom (55º37’N,1º37’W). Inner Farne and Brownsman provide breeding grounds for 1,957 breeding pairs (mean 2010-2014) of Arctic Tern Sterna paradisaea (Steel 2015). Canes have been used in an attempt to protect Arctic Terns from predation since 2002, but their effectiveness has not been quantified. Gulls breed on the Farne Island with approximate mean annual population size in breeding pairs: European Herring Gull Larus argentatus 1,600, Lesser Black-backed Gull Larus fuscus 1,600, Great Black-backed Gull Larus marinus 14 (Steel 2013, Steel 2015). There are no other avian and mammalian predators on the islands that we are aware of.

Experimental design

We used four experimental sites, three on Inner Farne and one on Brownsman (Supporting Information Fig. S1). Each site (A-C) on Inner Farne was a distinct breeding area for Arctic Tern and separated from other nesting sites by anthropogenic structures (Fig. S2). On Brownsman, the terns nest in large numbers in one colony (D). We divided each site into three treatments of approximately equal size: control, low density and high density canes (Fig 1). The location and order of the treatments was assigned randomly, except for site C where all of the terns settled within the caned treatments and the control area had to be increased to ensure it included tern nests. Note that site B was significantly larger than the other sites (Supporting Information Table S1).

The wingspan of the birds was used to determine the density levels of the canes. European Herring Gull and Lesser Black-backed Gulls have wingspans of approximately 1.2 - 1.5 m, compared with the smaller wingspan of the Arctic Tern at 0.75-0.85 m (Svensson 1992). In the low density treatment, the canes were spaced 2 m apart, with the next rows offset by 1 m, respectively. This allowed space for the full wingspan of a gull. In the high density treatments the canes were spaced 1 m apart (Fig.1), with a distance between cane ends less than the full wingspan of a gull but larger than that of terns.

After the first tern eggs were laid (14th May), we erected the canes (1.5 m in length), at an approximate angle of 70 to the ground. We monitored the sites using visual observation from a viewing platform for site B, and through the use of video cameras at sites A, C and D (Supporting Information Appendix S1). We used pilot observations between 14th May and the start of the experimental observations (20th May, see below) to decide on the times with highest gull activity that would then be our daily observation periods (Support Information Table S2). Using that information, we conducted daily observation periods of three hours per day, every day: after sunrise for a two-hour period and from the third to the second hour before sunset, between 20th May to the 6th of July 2016. These times were consistent across all sites.

At each site predation events by gulls were recorded per treatment and whether the predation was successful, for the above mentioned three-hour period per day. Due to practical limitations and the inaccessibility of sites, we were not able to record each site for the same number of days within the study period, however the treatments within each site were always monitored in the same way and for the same amount of time and thus are comparable.

A predation event was recorded when a gull walked into the site, presumably searching for eggs or chicks, or attempted to land in a treatment area. A successful predation was recorded if a gull took at least one egg or chick from a treatment area. If the same gull left the treatment area and then came back this was treated as another predation event.

We also used data provided by the National Trust to calculate nest density of the terns. Unfortunately the monitoring did not match our treatment sites, so we could only use proxies. Sites were monitored approximately every four days throughout the breeding season (Walsh et al. 1995), which provided the number of active nests at each site. We used the data collected up until the 7th June (first hatching date) to calculate the mean number of nests in the experimental treatments. After the chicks hatch, they move away from the nest scrape, therefore making them difficult to allocate accurately to a particular nest and area.

Statistical analyses

All analyses were run using R version 3.3.2 (R Core Team 2016). The dataset was not balanced and overall heterogeneous. Therefore, we used generalised linear mixed models (GLMMs) to test our hypotheses. These models generally deal well with heterogeneous datasets and thus produce more reliable estimates than traditional methods. Specifically, we used Bayesian Markov Chain Monte Carlo methods to estimate the parameter estimates, using the MCMCglmm package (Hadfield 2010). This method is especially well suited to heterogeneous data that are affected by over-dispersion (Hadfield 2010).

To assess whether the treatment affected the number of predation attempts, we used a Poisson error distribution. We used the number of predation attempts per 3-hours observation period as the response variable. The explanatory fixed factor was treatment, with control as the reference level. We modelled site and date as random effects on the intercept, because weather, gull nutritional demands, and tern egg and chick availability varied between sites and days, without losing additional degrees of freedom (Gelman & Hill 2007, Crawley 2013). To test whether the success of predation attempts was determined by the treatment we analysed the data on predation attempts. A binary variable indicating whether the attempt was successful or not was modelled as response variable, Treatment was a fixed explanatory factor and random effects were modelled as described above.

We considered a fixed effect to be statistically significant if the 95% credible interval (95CI, a Bayesian quasi-equivalent for confidence interval) for its coefficient did not overlap zero. We also report probability values (MCMC-P) to describe statistical significance of the fixed effects in the models. These refer to pMCMC as calculated in MCMCglmm, which is twice the MCMC estimate of the probability that the 95CI does not include 0 and can therefore be interpreted in a similar way to traditional P values.

RESULTS

In total, 319 hours of observation were logged over the four experimental sites, across a mean of 383 Arctic Tern nests. In total, 116 predation attempts were recorded, of which 41 resulted in a gull taking an egg or chick. The rate of predation ranged from 0.08–0.39 predation attempts per hour between treatments and sites, with predation rates noticeably higher in sites A and D, compared to sites B and C (Table 1).

Predation attempts

Caned areas received fewer predation attempts than the control areas, with approximately half the number of predation attempts over the experimental period (Table 2, Fig. 2). The effect of the canes was most pronounced in site D (Table 2).

When back-transformed, the model predicted a 52% reduction in the mean number of predation attempts in the high density caned areas (Table 2), and a 43% reduction in the low density caned areas (Table 2), compared to the control. However, note that the difference between the effect sizes of the high and low density caned areas was not statistically significant, as their 95CI overlapped considerably.

Success of predation attempts

In total, only 35% of predation attempts were successful and with the exception of site A, the proportion of successes seemed lowest in the high density caned areas (Table 1). However, this was not a statistically significant relationship (P > 0.39), which could be due to the low number of observed predation attempts, and the large variability between sites (Table 1).

DISCUSSION

Canes in the Arctic Tern colony reduced predation attempts by gulls on the Farne Islands. However, if an attempt took place, then canes did not affect whether it would be successful or not. Thus, while our data show promise for the use of canes as a conservation tool to reduce predation attempts, more research is needed.

There was considerable variation in the rate of predation per hour across sites. The variation could be due to the behaviour of individual gulls (Harris & Wanless 1997, Ackerman et al. 2014). Previous work indicates that gulls (Stickley et al. 1995) and other seabird species (Ronconi & St.Clair 2006, Soldatini et al. 2008) can become habituated to a deterrent over time. On our study site, canes have been used previously and therefore, some of the gulls could be habituated to this deterrent. However, the number of gulls that have not experienced canes as deterrent before may vary and is unknown, and could thus be another factor contributing to the observed variation between sites.

Gulls are largely generalist feeders, but some are known to specialise in predating conspecific and heterospecific eggs and chicks, and thought to be responsible for the majority of nest predation events in seabird colonies in the absence of other predators, including mammals (Sanz-Aguilar et al. 2009). Specialist individuals have a more targeted interest in a particular prey and might act differently towards the canes than generalist feeders. Therefore, the number of gulls specialising on nest predation in the vicinity of a site could also influence the success of the cane method.

Gulls have been known to target the easiest prey (Elphick & Rubega 1995) and this raises an ethical dilemma for conservation managers. If the canes act as a deterrent, placing them in a subset of colonies at risk might push predation onto species in areas without canes. Clearly, the next step is to test the effectiveness of canes over multiple breeding seasons and on multiple study sites, which would include potential issues arising from gulls habituating to cane treatment.

Our findings suggest that canes can reduce the number of gull predation attempts on Arctic Tern colonies. It is likely that the canes could provide a level of protection against predation for other ground nesting seabirds too. Going forwards, we need research to assess whether canes could impact reproductive output of ground nesting seabirds generally, and what the effects on gull population dynamics are as those require conservation management, too. Concluding, our study shows that inexpensive and easy-to-use canes can be effective in preventing gulls from attempting to predate on other ground-breeding seabirds, and future studies on the detailed impact on predators and prey are needed.

We would like to thank the National Trust for their hospitality and allowing carrying out this experiment on their land, Dr Hannah Dugdale for lending camera equipment and Anne Wilson for their knowledge of the islands and advice.

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SUPPORTING INFORMATION

Additional Supporting Information may be found in the online version of this article:

Appendix S1.

Table S1. Descriptions of the four sites.

Table S2. Numbers of individual Larus gulls flying across Inner Farne

Fig. S1. Map of the Farne Islands and study sites

Fig. S2. Photographs of the four study sites

Table 1. Summary of an experimental study quantifying predation by Larus spp. across four sites of breeding Sterna paradisaea and three treatments of cane density

Site

Treatment

Nest density

(mean ± SD)*

Observation time (hours)

Predation attempt rate (per hour)

Successful predation attempts (%)

A

Control

29.00 ± 17.96

103

0.20

28.57

Low density

21.60 ± 15.89

103

0.10

40.00

High density

84.40 ± 13.35

103

0.18

52.63

B

Control

70.00 ± 11.77

79

0.10

37.50

Low density

45.20 ± 12.22

79

0.05

25.00

High density

51.30 ± 39.41

79

0.08

16.70

C

Control

03.25 ± 02.36

63

0.02

100.00

Low density

04.00 ± 01.83

63

0.02

100.00

High density

08.75 ± 02.63

63

0.00

0.00

D

Control

23.45 ± 06.53

74

0.39

27.60

Low density

16.25 ± 08.47

74

0.16

50.00

High density

26.40 ± 11.37

74

0.07

0.00

*Mean nest density of Sterna paradisaea for the period 14th May-7th June 2016

Attempts

Parameter

b

95CI

p

Fixed

Intercept

-2.80

-4.57– -0.85

0.02

High density

-0.65

-1.17– -0.16

0.01

Low density

-0.84

-1.39– -0.29

0.002

Random

Date

0.42

0.00–0.92

Site

4.50

0.06–13.73

Residual

1.03

0.21–1.89

Table 2. Results from a Bayesian Poisson GLMM with the number of predation attempts of gulls on breeding tern colonies as response variable.

Figure legends

Fig.1 Placement of canes in i) low density treatment and ii) high density treatment, in four sites of Sterna paradisaea, to test the effectiveness at reducing predation pressure by Larus spp. Alternate rows of canes were angled towards the left and right at an approximate 70 angle to the ground.

Fig.2 Total number of predation attempts made by Larus spp. from four experimental sites of breeding Sterna paradisaea, showing both failed and successful attempts, across three treatments of cane density.

Figure 1.

Figure 2

Supporting Information

Table S1. Reference information regarding four experimental sites of breeding Sterna paradisaea used to test the effectiveness of canes as a deterrent against predation by Larus spp., on the Farne Islands in Northumberland

Site

Grid

reference

Area

(m²)

Max plant height

(m)*

Description

A

55º36’55”N

1º39’20”W

246

1.20

A walled area, with a lighthouse and cottage dividing the site into two sections. There was a small amount of bare ground (approximately 5%)*.

B

55º37’00”N

1º39’20”W

729

1.02

A walled area which was originally used as a vegetable garden. It contained approximately 30% bare ground*, but this was even across the three treatments.

C

55º37’01”N

1º39’20”W

258

0.92

A walled area, originally used as a cemetery. There was minimal bare ground (approximately 5%)*. Unlike the other sites, the canes were not placed out equally across the area (Control= 130m², Low density canes= 64m², High density canes= 64m²).

D

55º38’03”N

1º37’31”W

219

1.51

An un-walled area, which surrounds the east, west and north side of a cottage. All aspects received all three treatment. The amount of bare ground was different across the site (East= 15%, West= 40%, North= 20%)*.

*Estimation of bare ground (%) and plant height was recorded 5th June 2016 site D, and 6th June 2016 for sites A-C.

Date

No. of Larus spp. flying

Hours from dawn

Hours before sunset

0

1

2

3

4

3

2

1

22/04/2016

8

3

5

3

5

5

3

10

23/04/2016

12

13

9

5

3

8

5

5

24/04/2016

75

35

10

9

2

12

14

11

25/05/2016

25

28

11

6

5

16

10

13

26/05/2016

30

25

13

3

4

13

11

10

Total

150

104

48

26

19

54

43

50

Table S2. The number of individual gulls of the Larus genus which were seen flying across Inner Farne, in fifteen minute period, which started at hourly intervals in relation to dawn and dusk

Fig. S1. Location of the four areas of breeding Sterna paradisaea, used as experimental sites to test the effectiveness of canes at reducing predation pressure by Larus spp. The inset maps show the position of the Farne Islands in relation to the Northumberland coast and within the United Kingdom. Sites are indicated by filled circles (A- D). Lines on the maps represent geographical features (eg, rock, vegetation boundaries on Farnes, field margins, road on mainland).

Fig. S2. Four experimental sites on the Farne Islands, used by Sterna paradisaea in the breeding season, which were used to test the effectiveness of canes as a deterrent against predation by Larus spp.

Appendix S1

Camera trap set-up

We located optimal positions for cameras before S. paradisaea returned to the breeding site, to minimise disturbance to the birds. Three cameras (Little Acorn-6210) were needed at sites A and D to view the entire colony, and one (Bushnell Trophy Trail camera) in site C.

The cameras were set to take video recordings for the 3 hours of observation per day. In sites A and C, we left the cameras in position for no longer than 5 days before re-charging the batteries, collecting the data and putting them back into position. We only collected or returned the cameras to sites during dry weather, to avoid any adverse effects to the eggs or chicks (Walsh et al. 1995). Cameras were collected from site D approximately every 5 days, but this was dependant on tides and weather conditions. In site D cameras were placed into position by either CB or the National Trust Rangers.

2