Feeding ecology of wintering Great Northern Divers Gavia immer in Argyll, Scotland

Abstract

I investigated the role of tide and time of day on feeding success and prey species of the Great Northern Diver Gavia immer at its wintering grounds in Argyll, Scotland. Focal animal sampling was used on solitary divers to determine their activity during different tidal states and at different times across four main sites. When a bird performed ten successive dives, feeding success was recorded and inferred. Divers spent more time feeding early in the morning in comparison to other periods of the day. Less time was spent feeding at high tide, but this difference was not statistically significant. Divers spent 55% of the daylight period feeding, with most of this feeding time spent underwater. Prey was brought to the surface during 15% of dives and birds drank (thought to indicate ingestion of prey underwater) following 33% of dives. Crabs and flatfish were the main observed prey items, with 61% of the prey brought to the surface estimated to have a mass of less than 5 g, although items up to 80 g were consumed on occasion. The composition of prey brought to the surface varied between sites, with more crab prey items seen on sites with rocky substrates. I investigated differences in the feeding behaviour of adult and first-winter Divers at one site, but observed few differences. The importance of high-quality feeding sites for Great Northern Divers, and the implications of time spent underwater within current census techniques are discussed.