Prey Weight, Food Overlap, and Reproductive output of Potentially Competing Long-Eared and Tawny Owls

Abstract

Food niches of the Long-eared Owl Asio otus and the Tawny Owl Strix aluco were compared. The niche of the Tawny Owl was about three times wider than that of the Long-eared Owl. Both species, however, utilized about the same number of prey categories. When in the same area, the Tawny Owl took more water voles, birds, and amphibians and less field voles than did the Long-eared Owl. Food overlap between the two species was lower for neighbouring than for nonneighbouring owls. This strongly indicates that the species compete for food. Mean prey weights for the two owl species were positively correlated in different seasons, and were either equal for both owls, or higher for the Tawny Owl, during every season of the year. Reproductive output of Long-eared Owls increased with their distance from Tawny Owls' nests. This pattern is suggested to be caused by food competition. The opposite was found for the reproductive output of Tawny Owls. This seems to be the opposite of the prediction if competition is occurring, but could be caused by the fact that the Long-eared Owl is more likely to settle in a Tawny Owl territory in a year when this contains relatively abundant prey, than in a year when it does not. Therefore, an effect on the Tawny Owl would not be possible to demonstrate in this analysis. However, the competition seems to be asymmetrical.