Diet and nest attendance of incubating and chick-rearing northern fulmars (Fulmarus glacialis) in Shetland

We examined diets and nest attendance patterns of northern fulmars (Fulmarus glacialis) on Foula and Unst, Shetland, UK, during the breeding seasons of 1998 and 1999. Mean foraging trip duration, derived from nest attendance patterns, differed between incubation (32.5 h) and chick-rearing (early stage: 11.2 h, mid-stage: 20.4 h). It was influenced by chick age and obviously also by prey availability. The numbers of fulmars attending the colonies were influenced by wind direction, wind speed, time of day and state of reproduction. Diet samples were collected by regurgitations of adult and young fulmars. Diet was analysed by identifying prey items on the basis of otoliths, vertebrae, premaxillae (fish), cephalopod beaks and fragments of arthropod exoskeletons. Adult fulmars and chicks fed upon a wide range of prey types. Gadoid fish, including Norway pout (Trisopterus esmarkii), were the most common prey in the diet. Fish offal was found in 32% of regurgitates, clupeids in 15%. In contrast, the proportions of sandeels were very low (1%). This is different from previous studies where sandeels formed a large part of fulmar diet at Shetland in summer. Differences in food composition of incubating and chick-rearing fulmars were found for fish, but not for offal, crustaceans or squid. The percentage of non-discard fish (Clupeidae, Ammodytidae, Isospondylae) was significantly different between regurgitates from incubating and chick-rearing fulmars. Samples from chick-rearing adults contained non-discard items more frequently than regurgitates from incubating birds. The results strongly indicate that fulmars select to feed their chicks on energy-rich clupeids.