Breeding biology of Imperial Cormorant (Phalacrocorax atriceps) and Rock Shag (P. magellanicus) at Golfo San Jorge, Chubut, Argentina

Abstract

We studied the breeding biology of Imperial Cormorant Phalacrocorax atriceps and Rock Shag Phalacrocorax magellanicus at Golfo San Jorge, Argentina, from 1991 to 1993. Dates in which first eggs were observed varied among seasons, being between mid October and mid November for Imperial Cormorants, and between late October and early December for Rock Shags. During 1993, 87% and 86% Imperial Cormorant and Rock Shag pairs, respectively, completed their clutches within four weeks after egg-laying started. The number of eggs per nest recorded one month after the start of egg laying for both species was statistically different among years. The average number of eggs per nest was similar between Imperial Cormorants and Rock Shags during 1993 (2.4 vs. 2.1, respectively) but significantly different during 1991 (1.9 vs. 0.8). During two of the study seasons the volume of the third egg was significantly smaller than the first and second eggs. The average length of the incubation period for Imperial Cormorants was estimated at 27.9 days. Hatching success of Imperial Cormorants was 38.2% (1991) and 52.5% (1992). Chick weight increase and culmen growth showed maximum values during the second and third weeks after hatching, reaching asymptotic values at 60 days. Chick tarsus growth reached maximum values during the first week and asymptotic values at 30 days. During 1993 mean number of chicks fledged per nest was 1.13 for Imperial Cormorants and 0.83 for Rock Shags.