Influence of rodent density on nesting associations involving the Bar-tailed Godwit Limosa lapponica

The Bar-tailed Godwit Limosa lapponica rarely defends its nest aggressively against predators, but rather associates with other more aggressive specie s for protection. I studied the distribution of Bar-tailed Godwit nests rel ative to nests of the aggressive Long-tailed Skua Stercorarius longicaudus, as well as to nests of two slightly less aggressive alternative 'protectiv e umbrella' species, the Whimbrel Numenius phaeopus and the Grey Plover Plu vialis squatarola. Rodents are the main prey of the Long-tailed Skua but, i n years when numbers of rodents are low, Long-tailed Skuas also eat eggs an d chicks of other birds. Data on rodent densities and nest distribution wer e collected on Finnmarksvidda, Northern Norway in 1987, 1989, 1990, 1992 an d 1997, and on Taimyr, Siberia, in 1994 and 1995. It is concluded that the Bar-tailed Godwit is able to evaluate the probability of nest predation fro m Long-tailed Skuas, based on direct or indirect information about the stat us of local rodent populations. This information is used to choose between Long-tailed Skuas and other potential species for protection against nest p redators.