Selection of Prey and Success of Silver Gulls Robbing Crested Terns

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Abstract
Silver gulls (L. novaehollandiae) stole fish from crested terns (S. bergii) at One Tree Island, Great Barrier Reef, Australia. They attempted to steal larger fish of several kinds more often than smaller ones up to a point, but did not try to steal the largest fish most frequently. Silver gulls preferred disc-like fish 8-10 cm long and cone-like fish 14-16 cm long. The likelihood that gulls would try to steal a fish was influenced by its length, weight, shape and availability, of which weight appeared to be particularly important. Robbing success differed significantly with length in only 3 of the 11 types of prey available. Gulls robbed in various ways, the success of which depended on a tern''s maneuverability, method of evasion and speed of reaction to pirates.