Geographic Variation in Size and Reverse Size Dimorphism of the Great Horned Owl in North America

Abstract

Geographic variation in body size and reverse size dimorphism (RSD) of the Great Horned Owl (Bubo virginianus) was assessed from a suite of 18 variables measured on 418 specimens. Great Horned Owls varied considerably in size across both geographic and subspecific boundaries. In a comparison of regional means, the largest owls were from Ontario, the smallest from Texas and California, but strong support for Bergmann's Rule was absent. There was also no obvious relationship between geographic variation in body size of Great Horned Owls and the size of typical prey species. Reverse size dimorphism (RSD) was virtually constant across all regions. This lack of variation implies that RSD is a species-specific character which does not covary in a systematic way with body size. There was more geographic variation in the size of body core variables than there was for skull, leg, and wing characters. Great Horned Owls from southern localities had relatively small body cores providing weak support for Allen's Rule. Average body sizes and levels of RSD were calculated for seven subspecies (virginianus, occidentalis, wapacuthu [subarcticus], pallescens, pacificus, saturatus, and lagophonus). The largest subspecies is occidentalis, which is widely distributed in the mid-western and western U.S. The smallest is pacificus, which is restricted to California. Significant size differences were found between subspecies even where their ranges overlapped. This suggests either that gene flow across subspecific boundaries is restricted or that strong selection maintains the size differences. An evaluation of the genetic differences among subspecies is needed to assess the significance of racial variation in this species.