Wilson L.J., Daunt F. & Wanless S. (2004) Self-feeding and chick provisioning diet differ in the Common Guillemot Uria aalge. ARDEA 92 (2): 197-207

The Common Guillemot Uria aalge is one of the most abundant seabird species in the North Atlantic. The diet delivered to the chicks by breeding adults is well known, since the food is carried in the bill and readily observed at colonies. Much less is known about the diet of adults during the breeding season and, in the absence of empirical data, the diet of adults and chicks has generally been assumed to be the same. However, foraging theory predicts that adults should select higher quality prey for chick provisioning. Furthermore, Common Guillemots are single prey loaders, and likely to deliver large prey items for their chicks, whilst feeding themselves on a much broader range of prey sizes. In this paper, we present the first detailed comparison of adult and chick Common Guillemot diet, using data collected concurrently on both age groups at the Isle of May, south-east Scotland. Our results strongly support the prediction that breeding adults select higher quality prey for chick provisioning than for self-feeding. Chicks were primarily fed energy rich Sprats Sprattus sprattus and adult Lesser Sandeels Ammodytes marinus. In contrast, adults fed on smaller prey items, with young of the year Lesser Sandeels being the most frequently recorded. Seasonal variation in diet composition of both adults and chicks accorded well with our current understanding of the availability of different prey types, but the difference in prey size between self-feeding and chick provisioning persisted throughout the sampling period.