THE KLEPTOPARASITIC BEHAVIOUR OF THE ARCTIC SKUA STERCORARIUS PARASITICUS WITH THREE SPECIES OF TERN

The behaviour of Arctic Skuas when stealing fish from three species of tern was studied at the estuary of the River Ythan, Aberdeenshire. The skuas' attacks were made by flying low over the water at high speed and by approaching the terns from behind and below. The adaptiveness of this flight path is discussed. The skuas attacked singly and in groups of up to five. Group attacks were more successful than single bird attacks, but the return per bird was greatest when the skuas attacked in pairs. The success of attacks also depended upon the species of tern being attacked. The duration of attacks varied with the number of skuas in the attack, the species of tern being attacked and the outcome of the attack. Attacks were directed more frequently against Sandwich Terns than against Common Terns, correlating with the success of the attacks against these two species.