Demography of Black-necked Swans Cygnus melancoryphus in three Chilean wetland areas

Abstract

Numbers and distribution of Black-necked and Coscoroba Swans were poorly known until they were censused in Magallanes, southernmost Chile in 1971. This survey revealed a concentration of swans in this region with almost 11,000 Black-necked and 403 Coscoroba Swans. Recent censuses have focused upon a Black-necked Swan breeding population in Laguna El Peral (33°30'S), a 16 ha nature sanctuary (eight pairs breeding, maximum 50 migration), and Laguna Torca (34°46'S), a 65 ha nature reserve (11-18 breeding pairs, 80-210 migration). Competition for nesting sites appears to be the principal cause for a stable breeding population in these two areas. In the largest breeding area under study, Rio Cruces, (38°42'S; a 4,877 ha nature sanctuary), the total number of birds remained fairly stable at 800 birds from 1982-83 until 1985-86 when effective protection has led to a doubling of the population every season since. Breeders have increased from 90 to 250 pairs (1986-89). Between January and late May 1989 there was a sharp increase, from 2,178 to 6,426 birds. This was a result of immigration due to climatic drought in Argentina. Protection has been a key factor responsible for the increase of the breeding populations in the two study areas.