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Data on 12 factors presumed to influence the distribution of aggressive nest defence in I 1 1 species of waders (incubation-sharing by the parents, number of parents present near the nest, incubation time, nest habitat, breeding latitude, body mass, wing loading, wing structure, detectability on the nest, predator regime, coloniality and alternative prey) were collected from literature and field researchers. Body mass and number of parents present on the nest temtory (within response range when avian predators appear) explain 50% of the variation in aggressive defence behaviour. The results support the notion that ecological conditions l i e predation pressure are important in shaping wader parental care systems, with implications for mating systems. Altogether, the investigated factors explain around 70% of the variation in the samples. Future research on the level of individuals is suggested in order to explain the remaining variation.

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The Bar-tailed Godwit Limosa lapponica rarely defends its nest aggressively against predators, but rather associates with other more aggressive species for protection. I studied the distribution of Bar-tailed Godwit nests relative to nests of the aggressive Longtailed Skua Stercorarius longicaudus, as well as to nests of two slightly less aggressive alternative 'protective umbrella' species, the Whimbrel Numenius phaeopus and the Grey Plover Pluvialis spatarola. Rodents are the main prey of the Long-tailed Skua but, in years when numbers of rodents are low, Long-tailed Skuas also eat eggs and chicks of other birds. Data on rodent densities and nest distribution were collected on Finnmarksvidda, and 1995. It is concluded that the Bar-tailed Godwit is able to evaluate the probability of nest predation from Long-tailed Skuas, based on direct or indirect information about the status of local rodent populations. This information is used t o choose between Long-tailed Skuas and other potential species for protection against nest predators.

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