Food provisioning, diel activity, and home range size in the glaucous gull (Larus hyperboreus) on Bjørnøya

Abstract

The glaucous gull (Larus hyperboreus) is one of the only numerous predatory bird species on Svalbard. They are generalist feeders that hunt and scavenge for food along the coast and on the open sea. On Bjørnøya, the gulls mainly feed on carrion, avian prey, and fish. In this study, camera traps were set out on a total of nine glaucous gulls’ nests to capture images of prey delivered by the parents to their young. Some of the gulls were also fitted with Pathtrack® Nanofix + RF GPS tags that tracked their movement to study their habitat use and estimate home ranges. Fish and avian prey comprised the main part of the diet. The camera trap images showed that capelin (Mallotus villosus), guillemot (Uria sp.) chicks, and guillemot eggs were the main prey brought to the chicks. The composition of prey captured varied between nests, depending on location, with fish dominating the diet at nests situated closer to sea level. The gulls were not found to display a clear diurnal cycle, as also observed in other populations and species breeding in the Arctic. An interesting find in this study was that all four gulls fitted with a GPS tag had very small home ranges, only extending a few square kilometers around their nest site. The gulls rarely ventured out to sea and were never recorded more than a few hundred meters from land. With up to 30% of fish in some of the gulls’ diet, this led to the question of where and how the gulls caught the fish. Combining GPS data and prey delivery data from the camera traps, it was found that the gulls found most of the fish very close to their nest, suggesting that they mostly pirated fish from other seabirds or found shoals of fish very close to the island. To further strengthen the results in this study, more data and larger sample sizes is required, especially on the use of GPS tags in combination with camera traps, as this is still a novel data gathering method.