Gloger’s ecogeographical rule and colour variation among Willow Tits Parus montanus

Abstract

The Willow Tit Parus montanus is distributed across the Palaearctic and into the Oriental zoogeographical region with 15 subspecies. P. m. borealis is the only subspecies in Fennoscandia. Based on 86 skinned museum specimens from Norway (south of 64 oN), we found relatively large variations in the colours of the mantle, the underpart and the cheeks. Birds along the coast are significantly darker and have less white on their cheeks than inland birds. Furthermore, the birds become paler on their mantle, underparts and cheeks towards north and east, and in particular the annual precipitation explains a significant part of the colour variation. This is in accordance with Gloger's climatic rule that points out that feathers tend to be darkly coloured in habitats where relative humidity is high and pale where it is low.