In a southern Spanish marsh, black-headed gulls, Larus ridibundus, kleptoparasitized wintering wading birds feeding on a single prey species. The more frequently attacked species, in order of preference, were black-tailed godwit, Limosa limosa, little egret, Egretta garzetta, and black-winged stilt, Himantopus himantopus. The identity of the host species was the main factor affecting the robbing tactics of the gulls, primarily the number of gulls participating in the attack but also their selection by different age-classes of gulls. The foraging gains of gulls when attacking different species, together with the escape behaviour of the host species, determined whether the gulls spaced out and whether they attacked as a group. Competition among gulls affected the choice of a victim by young and adult gulls, with young gulls more frequently attacking the less profitable victims (stilts). © 1990 The Association for the Study of Animal Behaviour.

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