Predation or facilitation? An experimental assessment of whether generalist predators affect the breeding success of passerines

Predation has been suggested as an important factor affecting the success of breeding birds. Various studies have found that particular bird species either avoid or choose to breed in the proximity of predator nests. Here, we investigated whether the presence of a generalist predator, the Eurasian Kestrel, Falco tinnunculus, affects the breeding success of a passerine (House Sparrow, Passer domesticus), using an experimental design of nest box placement for the two species. No difference in breeding parameters was found with distance of House Sparrow nests from both active and inactive Kestrel nest boxes. However, whereas House Sparrows built nests and laid eggs in nest boxes both with and without active Kestrel nest boxes in the vicinity at a similar frequency, they laid more clutches, and the total number of eggs laid and young fledged during the breeding season was higher, in nest boxes located close to active Kestrel nest boxes than to inactive ones. Of the 367 House Sparrow nestlings banded, only 3 were found predated in Kestrel nests and no correlations were found between the percentage of birds in the Kestrels' diet and breeding parameters of the House Sparrows. The proximity of breeding Kestrels may facilitate some protection to House Sparrows from other predators, such as corvids, which are chased away by Kestrels near the nest.