Despite slight but significant differences in size of male and female Common Guillemots Uria aalge and previously published sex differences in parental behaviour, foraging ranges and diving behaviour, no evidence of sex-specific differences in choice or size of fish caught for chicks or for self-feeding was found among adults caught in a North Norwegian colony.

Numerous studies have addressed and documented sexual differences in various aspects of foraging behaviour and food provisioning that are not only common among sexually size-dimorphic birds but also among seabirds where this dimorphism is all but absent (Table 1 in Lewis et al. 2002). For example, among the Alcini (guillemots Uria spp., Razorbills Alca torda and Little Auk Alle alle) the sexes are only slightly dimorphic but parental roles are nevertheless skewed. This is most apparent at the end of the nesting period (during which both parents incubate and take care of the chick) when only the male takes the chick to sea and cares for it for several weeks (Gaston & Jones 1998; Harding et al. 2004). However, recent studies have revealed more subtle sex-specific differences among the Alcini with e.g. male Brünnich’s Guillemots Uria lomvia, Razorbills and Little Auks spending more time in the colony, foraging at different times of the day, diving deeper or having longer dive bouts than females (Jones et al. 2002; Paredes et al. 2008; Thaxter et al. 2009; Wojczulanis-Jakubas et al. 2009; Paredes & Insley 2010) and have hypothesised that such differences are related to the partitioning of parental roles during the chick development period. We know of only two studies, however, that have addressed sex-specific differences in prey delivered to chicks: Paredes et al. (2008) who studied Brünnich’s Guillemots and Razorbills and Thaxter et al. (2009) who studied Common Guillemots U. aalge, neither of which found significant differences. This paper carries Thaxter et al.’s (2009) study one step further by investigating not only sex-specific choice of prey brought to Common Guillemot chicks, but also testing whether adult diet is different during the chick-rearing period. Although otherwise very similar in overall body size, male Common Guillemots have slightly larger bills than females (Gaston & Jones 1998) and with the above-mentioned differences in foraging behaviour, one might predict that adult males and females differ in self-feeding diet. Whereas Gaston & Bradstreeet (1993) found no differences in diet among male and female Brünnich’s Guillemots in the Canadian Arctic, this is, as far as we know, the first study to address sex-specific differences in choice or size of fish caught for self- feeding among Common Guillemots.

We thank the Norwegian Coastal Administration for permission to use the lighthouse on Hornøya as our base for fieldwork, Tycho Anker-Nilssen (Norwegian Institute for Nature Research, Trondheim) for obtaining permission from the Norwegian Animal Research Authority to take stomach samples and Truls Moum (Bodø University College) for analysing the blood samples. Thanks too to Francis Daunt and Chris Thaxter for their helpful comments. The study was financed by the University of Tromsø and the Norwegian SEAPOP programme (www.seapop.no).

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