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How Owls Select Their Prey: A Study of Barn Owls Tyto alba and Their Small Mammal Prey

Male Barn Owls Tyto alba delivered significantly more male than female Field Voles Microtus agrestis to their mates and nestlings in spring. The male bias was evident both in the prey stored at the nest and in the skeletal remains of prey in pellets taken from the nest. The sex ratio in voles caught by trapping showed a slight female bias. Analysis of pellets produced by the male barn owls at their roosts at the same time, showed the same biased sex ratio of voles, demonstrating that selection of male voles occurred at capture and was not a result of differences between prey delivered to the nest and those consumed by the males. The mean weights of trapped male and female voles in spring were 23.4 g (SE 0.5) and 18.5 g (SE 0.4) respectively. During winter when voles were not breeding the sex ratio in pellets of male and female owls was close to unity. This suggests that the increased aggressiveness and/or activity levels of male voles associated with territorial behaviour and breeding in spring led to an increase in their detection rate by owls hunting by use of auditory cues. Thus the selection for male voles was the result of an increased encounter rate rather than optimal foraging based on the greater body mass of male voles. In spring the owls caught significantly more Field Voles than Wood Mice Apodemus sylvaticus than predicted from the relative abundances of the two species in the field. It is suggested this was the result of the greater ability of Wood Mice to detect the owls and their greater agility in evading capture.