Kleptoparasitism, the stealing of food items from other animals, is an important foraging strategy for many taxa. In many cases the kleptoparasites and hosts are different species but less commonly, prey items are stolen from conspecifics. Recent studies have highlighted the potential importance of intra-specific kleptoparasitism in food-stressed populations, and here we show facultative kleptoparasitism at two North Sea colonies of Common Guillemots Uria aalge during one year of poor breeding success and one year of near average breeding success. The frequency of kleptoparasitism differed between colonies and years, reflecting variation in the magnitude of the benefits from kleptoparasitism, which was probably associated with variation in prey availability at sea. Specialised foraging strategies such as kleptoparasitism may allow individuals to buffer adverse conditions more effectively than those foraging only at sea.

Kleptoparasitism, the stealing of food items from other animals, is a common behaviour recorded in many seabird species (Furness 1987). It can sometimes involve high energy expenditure and risk of injury (Furness 1987; Sirot 2000), and so may be beneficial to individuals only during food shortages (Oro 1996; Sirot 2000). Facultative kleptoparasitism occurs in many groups of birds and is an important feeding technique in several families of seabirds (Lavers & Jones 2007), although in some cases it may be a specialised behaviour shown in only a small proportion of individuals (Steele & Hockey 1995; Shealer & Spendelow 2002).

Common Guillemots Uria aalge (hereafter ‘Guillemots’) are not kleptoparasitic under normal feeding conditions but during periods of food-stress (e.g. during El Niño events), adults have been recorded stealing prey from small or satiated chicks (Ainley et al. 2002) and from other adults (Furness 1987). Recently, breeding success at Guillemot colonies bordering the North Sea has been poor (Mavor et al. 2005; Wanless et al. 2005; Heubeck 2009) and under these circumstances, kleptoparasites may profit from stealing food for their chicks. Changes in the social behaviour of this species have already been observed in response to poor food availability, with parents able to increase chick-provisioning rate by leaving their chick unattended at the colony and foraging simultaneously (Ashbrook et al. 2010). Here, we quantify the occurrence of kleptoparasitism under such conditions in two widely separated populations.

We thank Mark Newell, Francis Daunt and Sue Lewis for help and advice with Isle of May data collection, Scottish Natural Heritage for allowing us to work on the Isle of May National Nature Reserve and Larisa Lee Cruz for valuable discussion and comments. K.A. was supported by a NERC PhD studentship. Research was conducted in compliance with the appropriate Scottish Natural Heritage Licence. The seabird monitoring at Sumburgh Head was carried out under contract to the Shetland Oil Terminal Environmental Advisory Group, funded by the Sullom Voe Association Ltd.

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