Optimal foraging in chick-raising Common Guillemots (Uria aalge)

Abstract

The Norwegian population of the Common Guillemot Uria aalge has declined by > 95% since the 1960s, and is classified as critically endangered in the Norwegian Red List. Much of the recent decline has been attributed to reduced food availability, but without extensive documentation of adult diet. Instead, chick diet has been considered a proxy of adult diet during the breeding season in many Norwegian studies. Central place foraging theory, especially for single-prey loaders, however, predicts that this may not be so and this study compares directly the diet of adult and chick Common Guillemots during the breeding season at a colony in NE Norway. Whereas chicks were fed mainly capelin (Mallotus villosus) and sandeels (Ammodytes sp.), most of the adult diet consisted of the two youngest year classes of Gadidae, probably cod (Gadus morhua) and haddock (Melanogrammus aeglefinus). A successful ecosystem management of seabirds is dependent on a full understanding of how prey quality, abundance and availability influence seabird populations and their viability such that knowing the true diet of adult Common Guillemots has important implications in the modelling and management of the Norwegian populations.