Use of Space by Nesting Black-billed Gulls Larus bulleri: Behavioural Changes During the Reproductive Cycle

Abstract

Black-billed Gulls Larus bulleri nesting in seven colonies in braided rivers in New Zealand were studied in 1990. These habitats are subject to frequent floods following heavy rain or snow melts. While a colony contained only incubating birds, the gulls nested close together (mean nearest neighbour distance 45-80 cm in six colonies) but once chicks began to hatch and to wander into nearby territories, families required more space. Parents and families moved outside the original colony and, eventually, to the water's edge. Aggression levels were low for incubating birds, increased for parents with chicks still in the colony and decreased when parents with their chicks moved to the water's edge. Parents with territories at the water's edge benefitted from increased space, lower aggression rates, and decreased food piracy while they were feeding their chicks. For chicks, the advantages were decreased aggression from neighbours and the ability to escape to the water when a ground or aerial predator approached.