PIRACY BY LAUGHING GULLS LARUS ATRICILLA: AN EXAMPLE OF THE SELFISH GROUP

In 1969, 1500 pairs of terns Sterna spp. and 50 pairs of Laughing Gulls Larus atricilla nested on Petit Manan Island, Maine, U.S.A. The gulls formed groups of up to eleven individuals to steal fish from the terns. The start of piracy coincided with hatching of the gull's eggs. Larger groups more often succeeded in stealing fish than smaller groups. Group formation cannot be ascribed to rarity of terns to chase, and is not due to shared advantage in fish seized or time spent chasing (hence energy expended). A sufficient explanation is the individual advantage to some gulls that join a small group at the expense of the initiator. Individual gulls are likely to follow different strategies. Chasing appeared to be more readily elicited by a chase in progress than by a tern carrying a fish.