Breeding biology of the dolphin gull (Larus scoresbii) at Punta Tombo, Ar¬gentina

YORIO, P.; SWANN, S.; BOERSMA, D.

THE CONDOR

COOPER ORNITHOLOGICAL SOC

Año: 1996 vol. 98 p. 208 - 215

0010-5422

Dolphin Gulls (Larus [Leucophaeus] scoresbii) at Punta Tombo, Argentina, aggregated in small colonies of 16-24 pairs to breed. Colony location varied among years. Nesting density was 1.02 to 1.6 pairs/m2. Mean 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 2 to 5 days old they followed their parents away from the nesting area and, as they grew older, they began crêching 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 1984 to 1990 was variable, ranging from complete failure to 0.86 chicks surviving to 3 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 Penguins (Spheniscus magellanicus) and Imperial Cormorants (Phalacrocorax atriceps) fed their chicks. Dolphin Gulls also fed in the intertidal zone, stole cormorant and Kelp Gull (Larus dominicanus) eggs, and scavenged remains of penguin eggs already preyed upon by other species. They nested with other species and left the breeding area in February after the chicks fledged. They did not return to the breeding area in any numbers until September. The breeding biology of the Dolphin Gull appears to be largely influenced by their specialized scavenging habits and vulnerability to predation.