Sexual dimorphism in size of Tawny Owls (Strix aluco) : a method for sexing in field studies.

Abstract

Among owl species, sexual dimorphism in body size increases with increasing body size (Earhart & Johnson 1970) and so one would expect the Tawny Owl (Strix aluco L.) to exhibit a sexual size difference. The object of this study is to describe the sexual dimorphism in size in the Tawny Owl and to produce a practical method for sexing them in the field since they cannot be sexed on plumage differences. Discriminant analysis, which has been widely used to categorise statistically individual birds by paired measurements (Dunnet & Anderson 1961, Harris 1964, Harris & Hope Jones 1969, Corkhill 1972, Anderson 1975, Threlfall & Jewer 1978), was applied and the accuracy of classification determined.

A large sample of dead Tawny Owls, collected as part of a pollution monitoring programme throughout England and Wales (Stanley & Elliott 1976) were measured and sexed by dissection. The majority of specimens were road casualties since Tawny owls are particularly vulnerable to this hazard (Glue 1971). Live Tawny Owls were weighed and measured during intensive field studies on marked populations by Hirons (1976) and Hardy (1977).