Why Are the Ural Owl Strix uralensis and the Tawny Owl S. aluco Parapatric in Scandinavia?

Abstract

The Ural Owl and the Tawny Owl have ranges in Scandinavia which meet in central Sweden between 600N and 610N with only a small overlap. These two owl species were compared with respect to their food and habitat utilization within the narrow zone where sympatry occurs. Their food utilization, despite their conspicuous size difference, was nearly identical (a = 0.96), both species feeding mainly upon Arvicola terrestris and Microtus agrestis. Changes in prey composition between years were parallel. However, the Ural and Tawny Owls were clearly segregated by habitat (ca = 0.44) and thus did not compete directly. The Tawny Owl preferred arable land and the Ural Owl taiga forest with a large proportion of open habitats such as mires and clear-fellings. The present parapatric distribution border coincides closely with the sudden change in vegetation in central Sweden, where northern coniferous forest is replaced by a mixed farmland habitat. Partly due to modern forestry a large proportion of Ural Owl pairs lack suitable nest sites and produce no young. Thus, the distribution pattern is probably mainly due to differences in habitat preferance but may secondarily be an effect of nest site scarcity among Ural Owls. It is suggested that provision of abundant nest-boxes may lead to the Ural Owl extending its range into that of the Tawny Owl and possibly to intensified competitive interactions.