Breeding habitat requirements of the Imperial Cormorant and Rock Shag in Central Patagonia, Argentina

PUNTA, G.; YORIO, P.; SARAVIA, J.; GARCÍA BORBOROGLU, P.

WATERBIRDS

Año: 2003 vol. 26 p. 176 - 183

1524-4695

We studied the habitat requirements and nest site characteristics of Imperial Cormorants (Phalacrocorax atriceps) and Rock Shags (P. magellanicus) in northern Golfo San Jorge, Argentina. Colonies of both species were only located on islands. We characterized nesting habitat at 15 and 31 islands with and without Imperial Cormorant colonies, and 19 and 27 islands with and without Rock Shag colonies, respectively. The probability of finding nesting cormorants and shags was independent of island size. Both cormorants and shags nested on only 18% of islands or islets closer than one km to the mainland, but nested in between 35% and 52% of islands located further offshore. All Imperial Cormorant and Rock Shag colonies were located on bare, rocky substrate. All colonies of both species were located more than 50 m from shrubs. The mean slope of the substrate where colonies were located was significantly greater for Rock Shags than for Imperial Cormorants (73° vs. 3°). Orientation of cliffs and steep slopes where Rock Shags nested was mostly to the east. Over 86% of Imperial Cormorant colonies were located on the side of the island facing the open sea. Both height and depth of the nest were significantly larger for the Rock Shag, while internal diameter was significantly larger for Imperial Cormorant nests. The distribution patterns of cormorants and shags at the northern sector of Golfo San Jorge appears to be largely determined by the presence of islands, with availability of open rocky substrate and low shrub vegetation cover.