Spatial variation in the feeding ecology, foraging ranges, and breeding energetics of northern fulmars in the north-east Atlantic Ocean

Abstract

Northern fulmars (Fulmarus glacialis) are one of the most abundant birds in the North Atlantic Ocean and the two largest fulmar colonies in the UK are at St Kilda, Outer Hebrides and Foula, Shetland. These colonies are about 450 km apart and surrounded by waters that differ greatly in terms of potential food availability. Thus fish offal and whole fish discarded from whitefish trawlers are a major source of potential food in Shetland but not at St Kilda. Associated with this, previous studies have reported broad differences in diets and colony attendance patterns of adults at these two colonies, and have predicted better chick growth in Shetland than at St Kilda. More detailed information on differences between sites in the foraging behaviour and ranges of adults and the growth of chicks has not previously been available. This paper, therefore, presents detailed dietary analysis and uses land-based data to determine the durations and potential ranges of foraging trips by adults at the two colonies, feeding on different types of prey. Differences in the growth of nestlings at the two sites are described and explained, in terms of the body maintenance requirements of chicks, caloric density of food and variability in food provisioning rates by adults.