HOST SELECTION, ATTACK RATES AND SUCCESS RATES FOR BLACK-HEADED GULL KLEPTOPARASITISM OF TERNS

The behavioral ecology of Black-headed Gulls Larus ridibundus kleptopa rasitizing terns on Coquet Island, Northumberland, was investigated be tween 1992 and 1994. Black-headed Gulls demonstrated behavioral adapta tions that probably optimized returns for energy invested in chases. G ulls 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 rat e was higher. Sandwich Terns Sterna sandvicenis were selected preferen tially to Common Terns Sterna hirundo, which were selected more freque ntly than Arctic Terns Sterna paradisaea. This selection was independe nt of fish size; there was no difference in success rate between speci es. 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 s uccess: most gulls made only a single attack on a host. Black-headed G ulls often attacked in groups. Larger group size was associated with i ncreased success, although the estimated average energy return for eac h gull was lower than for gulls foraging individually. By joining in c hases initiated by another, gulls may reduce costs of chasing and be i n a better position than the initiator of the chase to collect the foo d once it is dropped.