EFFECTS OF AVIAN MOBBING ON ROOST USE AND DIET OF POWERFUL OWLS, NINOX STRENUA

We observed the species and numbers of mobbing birds and their effects on a large, nocturnal, bird-eating predator, the powerful owl, togeth er with the pattern of owl predation on mobbing and non-mobbing specie s. Owls were mobbed on 35 occasions by seven of 44 species of forest b irds at a site composed of open forest (88% by area) and rainforest (1 2%). The majority of bouts involved individuals of a single species, a lthough mixed groups were observed on nine occasions. Regular mobbers were between 4 and 26% of the owls' body weight. Owls abandoned their daytime roosts during 26% of bouts and responded by calling or activel y monitoring mobbers during 54% of bouts. Mobbing appeared to explain why owls roosted in rainforest significantly more often than expected by its availability, mobbing being significantly less frequent in rain forest than in open forest. Only one mobbing species regularly occupie d rainforest and the canopy of roosts in rainforest was denser than th at in open forest, thus reducing the chances of an owl being detected by potential mobbers. Twelve species of forest birds were within the r ange of prey size of the powerful owl (75-800 g): six were mobbers and six non-mobbers. The frequency of owl predation on non-mobbers was 8. 75 times that on mobbers. The species in this study took a high risk b y mobbing a very large predator, but benefited by greatly reducing the ir chances of predation. (C) 1998 The Association for the Study of Ani mal Behaviour.