Beyond habitat: effects of conspecific and heterospecific aggregation on the spatial structure of a wetland nesting bird community

Nest location is a key factor influencing reproductive success in birds, and habitat choice is considered the main way in which birds select nest sites. Less attention has been devoted to the demand for proximity to other bird nests, which can provide additional profit, namely defense against predators. Here we analyzed the contributions of habitat, and conspecific and heterospecific aggregation to the spatial arrangement of breeding birds in a model bird community. We surveyed a pristine Siberian wetland bird community with the aim to locate all bird territories or nests, in 1993 and 2013. Habitat explained much of the nest site choice, but the nests were aggregated both intra- and inter-specifically more than the spatial pattern of the habitat could explain. In particular, ducks, grebes and some waders bred nearby the most abundant active nest defenders, such as gulls and terns. Heterospecific associations were particularly pronounced in 2013, when the community was impoverished and one common active defender (white-winged black tern Chlidonias leucopterus) was replaced by a less numerous but aggressive predator (Mongolian gull Larus mongolicus). The results suggest that spatial pattern in bird nests may be influenced by the (dis)appearance of one or a few species, which can play a role as umbrella or predator species. Integration of factors supporting the breeding of umbrella species, such as gulls, may became key targets for comprehensive conservation measures in large wetlands.