BREEDING BIOLOGY OF THE DOLPHIN GULL AT PUNTA-TOMBO, ARGENTINA

Dolphin Gulls (Larus [Leucophaeus] scoresbii) at Punta Tombo, Argentin a, aggregated in small colonies of 16-24 pairs to breed. Colony locati on varied among years. Nesting density was 1.02 to 1.6 pairs/m(2). Mea n clutch size was 1.9 and 2.0 in 1987 and 1989, respectively, and egg volume was similar among years. incubation took between 24 and 27 days , with chicks hatching from mid to late December, When chicks were two to five days old they followed their parents away from the nesting ar ea and, as they grew older, they began creching in one or more groups. Egg mortality in 1987 and 1989 was 41% and 46%, respectively, mainly through disappearance and presumed predation. Breeding success from 19 84 to 1990 was variable, ranging from complete failure to 0.86 chicks surviving to three weeks of age per active nest. After nest losses in 1986 and 1990 birds renested. During the breeding season Dolphin Gulls were specialized scavengers, mainly eating southern sea lion (Otaria flavescens) excrement and scraps of food dropped when Magellanic Pengu ins (Spheniscus magellanicus) and Imperial Cormorants (Phalacrocorax a triceps) fed their chicks. Dolphin Gulls also fed in the intertidal zo ne, stole cormorant and Kelp Gull (Larus dominicanus) eggs, and scaven ged remains of penguin eggs already preyed upon by other species. They nested with other species and left the breeding area in February afte r the chicks fledged. They did not return to the breeding area in any numbers until September. The breeding biology of the Dolphin Gull appe ars to be largely influenced by their specialized scavenging habits an d vulnerability to predation.