COLONIAL BREEDING AND NEST PREDATION IN THE JACKDAW CORVUS-MONEDULA USING OLD BLACK WOODPECKER DRYOCOPUS-MARTIUS HOLES

Jackdaws Corvus monedula breeding in old holes of the Black Woodpecker Dryocopus martius were studied during 6 years in central Sweden. Of 1 31 breeding attempts, only 56% were successful. Nest predation caused 74% of the breeding failures. The most common predator was probably th e Pine Marten Martes martes, as fresh hair was found in 42% of the nes ts whose contents had been predated. Predation risk of nest contents w as not related to the height above ground, but it was significantly re duced with increasing colony size. Since Jackdaws are known to communa lly defend their nests, a likely explanation is that the Jackdaw benef its from group defence against predators. During the study, the number of breeding Jackdaw pairs in Black Woodpecker holes declined, probabl y linked with a recent increase in the numbers of Pine Martens in the area.