Parental defence in Long-eared Owls Asio otus: effects of breeding stage, parent sex and human persecution.

The responses of male and female Long-eared Owls to a human approaching the nest were examined. Each nest was only tested once. Nest defence increased significantly throughout the breeding season because older chicks were defended more strongly than younger chicks and eggs. No correlations were however found between defence intensity and laying date, clutch- or brood-size. These results generally do not support either the renesting-potential hypothesis or the positive reinforcement hypothesis but are in accordance with both the age-investment hypothesis and the vulnerability hypothesis, females defended nestlings more often and more vigorously than did males. Given division of labour between sexes and the related reversed sexual dimorphism in size, female Long-eared Owls may be more willing to engage in nest defence simply because they are always nearer eggs and chicks during all breeding stages and are larger/heavier than males; consequently they may allocate more time and energy to this activity. Finally, owls experiencing higher levels of human persecution took smaller risks when defending nests than owls breeding in an undisturbed area.