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.