Predation on Ground-Nesting Birds' Nests in Relation to Predator Densities and Habitat Edge

Abstract

To study which predators are responsible for nest-losses among ground-nesting birds in boreal forest and how predators utilize different habitats, especially forest/farmland edge, artificial nests were exposed to predators in central Sweden in 1981. A setup which forced nest-robbers to leave foot-prints showed that different avian and mammalian species robbed nests in relation to their relative densities. Among both avian and mammalian nest-robbers there were habitat specialists and generalists. Predators appeared not to develop a memory for the experimental nests. In forest, birds were relatively more common than mammals as nest robbers than in farmland. Differences in the role of different predator species as nest robbers in forest vs. farmland habitats reflected their choice of habitat. No predator seemed to have developed specific nest-robbing skills and loss of individual nests was considered as a random event. Predation rates were higher when nests could be detected from a distance. However, the relevance of the results for real nests remains largely unknown. Variations in predation pressure in relation to different types of ecotones is discussed in view of the recent debate on how patterns of predation become altered as the size of patches of pristine environments are reduced as a consequence of human influences. It is suggested that the main factor affecting the rate of predation in patchy environments is the steepness of productivity gradients between an habitat island and the surrounding matrix rather than patch size itself.