Host Selection, Attack Rates and Success Rates for Black-Headed Gull Kleptoparasitism of Terns

The behavioral ecology of Black-headed Gulls Larus ridibundus kleptoparasitizing terns on Coquet Island, Northumberland, was investigated between 1992 and 1994. Black-headed Gulls demonstrated behavioral adaptations that probably optimized returns for energy invested in chases. Gulls selectively attacked terns carrying larger fish, probably because the energetic content of a fish increases exponentially with length. They also selected terns carrying more than 1 fish, as the success rate was higher. Sandwich Terns Sterna sandvicensis were selected preferentially to Common Terns Sterna hirundo, which were selected more frequently than Arctic Terns Sterna paradisaea. This selection was independent of fish size; there was no difference in success rate between species. Host species selection may be due to the relative proximity of the different tern colonies to the gull colony. The number of swoops made during an attack was associated with a decrease in the likelihood of success: most gulls made only a single attack on a host. Black-headed Gulls often attacked in groups. Larger group size was associated with increased success, although the estimated average energy return for each gull was lower than for gulls foraging individually. By joining in chases initiated by another, gulls may reduce costs of chasing and be in a better position than the initiator of the chase to collect the food once it is dropped.