Foraging strategies of the black-legged kittiwake Rissa tridactyla at a North Sea colony: evidence for a maximum foraging range

Abstract

Black-legged kittiwakes Rissa tridactyla on the Isle of May, southeast Scotland, feed predominantly on the lesser sandeel Ammodytes marinus, an abundant, pelagic fish that is currently the subject of the largest fishery in the North Sea. The population of black-legged kittiwakes on the Isle of May is declining, and the fishery has been implicated. In order to assess this concern, there is an urgent need to improve our understanding of the factors that affect black-legged kittiwake forag- ing behaviour. During 1999, we carried out a detailed study of the foraging strategies of black-legged kittiwakes using purpose-built activity loggers that allowed us to distinguish 4 key behaviours: travelling flight, foraging flight, presence on the sea surface and attendance at the nest. We used the data to model 2 key aspects of time allocation at sea: (1) the relationship between the travelling time and trip duration and (2) the ratio of time spent actively foraging to time of inactivity on the sea sur- face at the foraging grounds. We found that a broken-stick model with a flat asymptote was the best fit for the relationship between travelling time and trip duration. Using published flight speeds for this species, we calculate that breeding black-legged kittiwakes on the Isle of May had a maximum range of 73 ± 9 km from the colony. We speculate that this upper limit is dictated by the distribution of prey rather than any energetic constraint on flight costs: a large sand bank complex, known to have high concentrations of lesser sandeels, lies entirely within this range. There was no consistent pattern in the ratio of the active to inactive components of the foraging trip, suggesting that this species exhibits highly flexible foraging strategies at sea, probably reflecting the patchy and un- predictable distribution and availability of its prey. Our findings suggest that the birds are feeding on sandeels at the same time and in the same area as the operations of the sandeel fishery.