Individual tracking of an arboreal rodent shows that predation on cavity-nesting birds is not incidental

Abstract Nest predation is the leading cause of breeding failure in birds. A crucial aspect in predators’ foraging ecology is whether they depredate bird nests incidentally or intentionally, and if they develop searching mechanisms to locate bird nests. Here we present data on individually transponder-tagged Edible dormice ( Glis glis ), an arboreal rodent that uses tree cavities as den sites and depredates on clutches of cavity-nesting birds. During nine breeding seasons, we deployed automatic reading devices on wooden nest boxes that both breeding birds and dormice use. By recording dormice visits to cavities with active bird nests, we provide detailed data on the identity of dormice as nest predators. We recorded 23 cases of 18 individual dormice visiting the nest boxes while there were nesting birds. In 18 cases, the encounter led to nest predation. All nest predation events occurred during the night. Males were much more common predators than females. For five dormice, we have records of repeated visits to occupied bird nests (mostly titmice and flycatchers). All cases were in the same year; no dormice were recorded as predators repeatedly between the years. In two cases, the dormice depredated both visited nests; in three cases, the dormice did not depredate the nest during the first visit but did so on the second visit. Our data show that the dormice make frequent visits to all available cavities and that most predation events were likely not incidental.