Range Extensions of Some Boreal Owl Species: Comments on Snow Cover, Ice Crusts, and Climate Change

Recent observations have documented that some boreal owl species in Europe have made unexpected eruptive movements and some have extended their distribution, among them the Great Grey owl (Strix nebulosa). Based on published data, it can be assumed that both the numbers and distribution have varied considerably in the past 120 years. In the Finnish population, for example, there has been a clear southward shift in range— from Lapland toward the central and eastern regions (Sulkava and Huhtala, 1997). In Sweden, the Great Grey owl is distributed throughout the boreal zone but is most frequently found in the northeasternmost parts of the country (Hipkiss et al., 2008). In 2010–2012, the species was found nesting in unprecedented numbers in southeastern Norway (Solheim, 2009, 2014a). Record-breaking numbers of breeding individuals were reported in Sweden and Norway in 2010 and 2011, followed by 4105 observations in 2012 as revealed by the National Species Archives in Sweden (Solheim, 2014b). In the first half of the last century, the species was known as a rare breeder only in northernmost Norway (i.e., in Pasvik, Finnmark) (Hagen, 1989; Sulkava and Huhtala, 1997). Today it is a regular breeder over a considerable part of the southeast forested area in the country (Solheim, 2014b). Other extensions have been noted as well. In 2007–2009, the species was found in Belarus near the Polish border, southwest of its regular breeding grounds, and the species recently has been visiting many other areas (Ławicki et al., 2013). Rapid range extensions and population movements are so marked that they indicate a large-scale ecological change. Is it caused by climate change and global warming?