Competition for Nest Sites between Kelp Gulls (Larus dominicanus) and Terns (Sterna maxima and S. eurygnatha) in Patagonia

Seabirds frequently nest in mixed-species colonies (Burger 1985). Several studies have shown differences in habitat and nest-site use among seabirds (Buckley and Buckley 1980, Croxall and Prince 1980, Fasola and Canova 1992). However, some degree of overlap in habitat and nest-site requirements may exist between species, resulting in interspecific competition that often leads to nest-site displacement or even the replacement of one species by another (Buckley and Buckley 1980, Burger 1985). Spatial interactions between seabirds can be more intense in areas where habitat is limited or when the population of one of the species is expanding (Burger and Shisler 1978, Trivelpiece and Volkman 1979, Duffy 1983, Ainley 1990). At many mixed colonies of larids, larger species often displace smaller species from their territories, occasionally forcing them to abandon the breeding area (Burger and Shisler 1978, Burger 1985). In addition, larger larids generally arrive at breeding sites earlier (Morris and Hunter 1976, Burger 1985), which gives them a competitive advantage over smaller species. Gulls (Larus spp.) frequently have been involved in the displacement of other species from breeding habitats (Thomas 1972, Burger 1979, Bradley and Monaghan 1986, Furness and Monaghan 1987). Terns appear to be one of the most affected seabirds, and some tern populations have decreased as a result of population expansion by gulls (Thomas 1972, Nisbet 1973, Greenhalgh 1974, Courtney and Blokpoel 1983, Parnell et al. 1988, Ainley and Hunt 1991, Blokpoel and Scharf 1991). At Punta Le6n, Argentina, Royal Terns (Sterna maxima) and Cayenne Terns (S. eurygnatha) breed intermingled in a dense cluster of nests within a Kelp Gull (Larus dominicanus) colony. The Kelp Gull colony increased in size from 3,200 pairs in 1982 to 6,500 in 1995, and gulls currently occupy almost all of the available nesting space (Yorio et al. 1994). Given the overlap in nest-site use and the current expansion of Kelp Gulls, we expected that competition for nesting space could result in negative effects on both species of terns. In this paper, we document spatial interactions between Kelp Gulls and Royal and Cayenne terns at Punta Le6n, Patagonia. We