The aim of the study was to describe Yellow-legged Gull Larus michahellis chick diet in the southeast Bay of Biscay. Sampling was carried out in three colonies (Ulía, Santa Clara, Guetaria) in Gipuzkoa, northern Iberia, during the breeding seasons of 2007 and 2008. Regurgitates (n = 70) were collected from chicks of c. 20 days old when they were ringed. Overall, 79 prey items were found, and each item was weighed and classified into one of the following prey-groups: marine (fish (Pisces), crab (Brachyura)), refuse tips (pork, beef, rabbit, chicken), terrestrial (earthworms (Lumbricinae), molluscs (Mollusca), insects (Insecta)), others (mainly vegetables or unidentified prey). The proportion of each prey-group did not differ from one year to the next and most prey was of marine origin (59.5%; all fish except for one crab), followed by refuse tips (22.8%), terrestrial (13.9%), and others (3.8%). These findings concur with those at colonies in northwest Iberia, and while local exceptions might occur, Yellow-legged Gull chick diet in northern Iberia is predominantly marine fish.

Human activities often have an impact on population size of wild vertebrates, and many opportunistic species have adapted to exploit some superabundant food resources generated by man (Garrott et al. 1993). This phenomenon is well known in gulls, and populations of several species have increased in response to anthro- pogenic factors (e.g. Steele & Hockey 1990; Skorka et al. 2005; Duhem et al. 2008).

With a population of some 200,000 breeding pairs, the Yellow-legged Gull Larus michahellis is one of the most abundant large gulls in Europe (Olsen & Larsson 2004). Nearly 100,000 pairs breed in Iberia, where the species has shown a notable population increase during the second half of the twentieth century (Bermejo & Mouriño 2003; Arizaga et al. 2009). Such an increase can generate problems, due to unsanitary (Monaghan et al. 1985; Ramos et al. 2010), safety (Brown et al. 2001), ecological (Rusticali et al. 1999; Vidal et al. 2000; Oro et al. 2005; but see Oro & Martinez-Abrain 2007) or social impacts (Raven & Coulson 1997). Consequently, the species has been the target of management policies, often based on culling that normally have a null or very short-term effect on population trends (e.g. Bosch et al. 2000). Diet analyses can be of key importance from this standpoint, because they provide quantitative data on which colonies depend on which prey types (Ramos et al. 2006; Ramos et al. 2009), and breeding success is largely determined by diet (Annett & Pierotti 1999).

The Yellow-legged Gull is able to forage on a wide spectrum of prey, from marine prey to refuse tips, and including freshwater prey, crops or earthworms (Lumbricinae) (Álvarez & Méndez 1995; Munilla 1997; Ramos et al. 2006; Moreno et al. 2009). In Iberia, the diet has been studied mainly at colonies in the Mediterranean, and in Galicia, northwest Iberia. Although fish (Pisces) are one of the preferred resources during breeding (Ramos et al. 2006; Moreno et al. 2009), some colonies depend highly on refuse tips (Bosch et al. 1994; Ramos et al. 2009). Geographic variation in diet is correlated with the distance of colonies to important feeding locations, such as harbours, fishing areas, refuse tips or crops (Moreno et al. 2009; Ramos et al. 2009). Distribution of food at a local scale hence determines the chief food items for given colonies. In northwest Iberia, the swimming crab Polybius henslowii was found in nearly 40% of regurgitates from adult Yellow-legged Gulls (Álvarez & Méndez 1995; Munilla 1997), but was much less important for chicks (Moreno et al. 2009). The diet for other areas of northern Iberia is unknown, and the aim of this study is hence to analyse the extent to which Yellow-legged Gulls breeding in the southeast Bay of Biscay depend on refuse tips or other food resources to provision their chicks.

This research was funded by Diputación de Gipuzkoa and the Basque Government. Thanks to the people who assisted with field work, especially members of the Txingudi Ringing Station. C. Escala and M. Larraz (University of Navarra) helped to identify the taxa of some fish and mollusc species, while X. Esparza and O. Frías assisted with prey identification. S. Garthe, M. Heubeck and R. Ramos provided very valuable comments that helped us to improve an earlier version of this work.

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