INTERSPECIFIC COMPETITION FOR NEST HOLES CAUSES ADULT MORTALITY IN THE COLLARED FLYCATCHER

Interspecific competition for nest sites between tits (Parus spp.) and Collared Flycatchers (Ficedula albicollis) was responsible for the de ath of 23 flycatchers during one breeding season. Eighteen (78%) of th e killed flycatchers were males, and nineteen (83%) of the kills took place in nest boxes occupied by Great Tits (Parus major), and two (9%) in boxes occupied by Blue Tits (P. caeruleus). The number of casualti es in different plots increased with increasing proportion of nest box es occupied by tits (r = 0.76), and decreased with increasing density of nest boxes (r = -0.43). The number of casualties equalled up to 17% of all flycatcher individuals breeding in a given plot, indicating th at interspecific competition for nest sites may constitute a significa nt source of adult mortality in the Collared Flycatcher population stu died.