Identifying habitat-driven spatial variation in colony size of Herring Gulls Larus argentatus

Figures & data

Figure 1. Study region (in grey) where spatial variation of changes in Herring Gull colony sizes were assessed between the first survey in 1969 and the most recent survey from 1998 to 2002. The study region spans two regional seas: Minches and West Scotland in the north and Irish Sea in the south (JNCC 2014). Also indicated are the four ICES sea areas (45, 46, 47 and 50) used to estimate availability of fishery discards.

Figure 2. Locations of Herring Gull colonies with known changes in colony size between 1969 and 2002 across the study region. The shade of the circle depicts the extent of colony growth rate over the study period; along a gradient of white (strongest increase) to black (strongest decline); categories based on a K-means cluster analysis (see text). The size of the circle reflects the size of the colony at the end of the census period during Seabird 2000 (1998–2002).

Table 1. Rank of general linear regression models explaining variation in Herring Gull colony size in Seabird 2000 by proxies of the availability of different foraging habitats using Akaike's Information Criterion corrected for small sample size (AICc). k is the number of estimated parameters included, w_i is the Akaike weight, and ΔAICc is the AICc difference. Top-supported models (ΔAICc < 2) are shown in bold.

Variables included within model	k	AICc	ΔAICc	wi
Farmland, wave fetch, fishery CPUE	4	302.0	0.00	0.435
Farmland, wave fetch	3	303.6	1.61	0.195
Farmland, wave fetch, built-up, fishery CPUE	5	304.4	2.41	0.131
Farmland, wave fetch, built-up	4	306.0	3.91	0.062
Farmland, fishery CPUE	3	306.7	4.62	0.043
Wave fetch, built-up	3	307.0	5.01	0.036
Wave fetch, built-up, fishery CPUE	4	307.8	5.76	0.024
Farmland, built-up, fishery CPUE	4	308.1	6.05	0.021
Built-up, fishery CPUE	3	308.5	6.42	0.018
Built-up	2	309.1	7.04	0.013
Farmland	2	310.0	7.96	0.008
Farmland, built-up	3	310.6	8.55	0.006
Wave fetch	2	310.9	8.85	0.005
Wave fetch, fishery CPUE	3	312.7	10.69	0.002
Intercept only	1	314.7	12.62	0.001
Fishery CPUE	2	315.6	13.57	0.000

Table 2. Final model-averaged parameter estimates with standard error and 95% confidence intervals. Effects are model-averaged slope estimates derived from the models in Table 1, ranked by relative importance of each habitat variable to explain variation in Herring Gull colony size in Seabird 2000.

Parameter	Effect	SE	95% confidence intervals		Relative importance^a
			Lower	Upper
Intercept	2.4109
Farmland	0.0012	0.0004	0.0005	0.0020	0.90
Wave fetch	−0.0018	0.0007	−0.0031	−0.0005	0.89
Fishery CPUE	0.00063	0.00057	−0.00002	0.00184	0.67

^aRelative importance refers to the sum of all model weights in which a given parameter occurs.

JNCC. 2014. UK Regional Seas. www.jncc.defra.gov.uk/page-1612.

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