Habitat and nest site characteristics of Olrog's Gull Larus atlanticus breeding at Bahia San Blas, Argentina

Olrog's Gull Larus atlanticus is a vulnerable species endemic to the Argent ine Atlantic coast. We present information on new breeding colonies, update information on known colonies, and describe habitat and nest site characte ristics of Olrog's Gulls breeding at Bahia San Bias, southern Buenos Aires Province, Argentina. information was obtained during the 1998 breeding seas on. We recorded four colonies on islets, with a total population size of 30 5 pairs. Colonies were located on open ground, partly bordered by vegetatio n. The two colonies at Islote Arroyo Jabali Oeste were located more than 15 0 m from the high tide Line, while the two colonies at Banco Nordeste were placed only a few metres from the high tide mark (1-6 m). Most nests were l ocated more than a metre away from low Atriplex patagonica bushes (20-30 cm high) and herbaceous species, although a few nests were a few cm from vege tation. Nests were built on vegetation debris, on low herbaceous vegetation , and on gravel and sand. All nests at the four colonies were built with ve getation debris, mainly Spartina densiflora and some Salicornia sp. Mean ex ternal and internal diameters of nests were 40.57 +/- 5.14 and 19.71 +/- 2. 21 cm, respectively. Gulls nested in discrete groups at relatively high den sities, which ranged between 0.62 and 1.1 nests/m(2). Nest occupation and n umber of eggs per nest varied among colonies. Kelp Gulls Larus dominicanus were observed nesting in close proximity to all colonies. Further studies a re needed to assess habitat partitioning between these two species, particu larly considering that they nest in association at all breeding locations i n coastal Argentina and that Kelp Gulls have shown an important population increase in recent decades.