Nest Predation and Nest Site Choice in Passerine Birds in Habitat Patches of Different Size: A Study of Magpies and Blackbirds

Nest predation and anti-nest predation tactics of passerine birds was studied in patches of elm tree (Ulmus glabra) habitat in Danish farmland. Nest predation rate on blackbird (Turdus merula) nests with either real or dummy blackbird eggs decreased with increasing area of habitat patches. Presence of a breeding pair of magpies (Pica pica) increased predation rate irrespective of habitat patch size. Passerine birds with open nests were relatively more abundant in large habitat patches. This, however, was not caused by a low abundance of magpies. Habitat patches, where magpies nested only in some years, did not hold more open nesters in years when magpies were absent, than when magpies were present. Blackbirds disproportionately often built their nests in sites with low predation rates (inside buildings, in coniferous trees, in hedges) in small as compared with large habitat patches. But blackbirds did not build their nests in such concealed sites more frequently in habitat patches with magpies than in patches without breeding magpies. Habitat features associated with elevated risks of nest predation thus affected the nest site choice of blackbirds more than did the actual presence of predators.