Erritzøe, J. & Fuller, R.A. 1999. Sex differences in winter distribution of Long-eared Owls (Asio otus) in Denmark and neighbouring countries. Vogelwarte, 40, 80–87.
Evidence presented here indicates that in western Denmark, the sex ratio of Long-eared Owls (Asio otus) during the winter months is significantly biased towards females. Museum collections revealed the same phenomenon in the Netherlands and northern Germany, while sex ratios approach parity in eastern Denmark, Sweden and central and southern Germany. From the literature, it is clear that there is also a significant winter surplus of females in England, Scotland and the Shetland Islands. Present hypotheses for differential migration are reviewed and it is concluded that none explain adequately this pattern. An alternative hypothesis is presented here, in which heavy predation pressure by Eagle Owls (Bubo bubo) and Goshawks (Accipiter gentilis) on the breeding range in Fenno-Scandia and parts of these birds’ wintering range in northern and western Europe is the primary cause of the differential in the observed pattern of winter distribution.