Size, sexual size dimorphism, and their measurement in Great Horned Owls in Alberta

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Abstract
The degree of reversed size dimorphism between male and female Great Horned Owls in Alberta was examined by measuring 18 skeletal and 7 external characters on museum specimens. Females are significantly larger than males on all characters except skull width, which is larger in males. The degree of sexual size dimorphism is highest for flight characters and weight and weaker for skull, body core, and lower limb elements. High variances associated with external characters make the use of Storer's dimorphism index an unreliable measure of between-sex differences. External characters show weak intercharacter correlations and are poorly correlated with size as defined by first principal component scores obtained from skeletal characters. Despite significant differences between an ideal size axis and eigenvectors of both correlation and covariance matrices, the three are shown to be equivalent in estimating size. Inequality among character variances bias the interpretation of covariance-based principal component analyses supporting the use of correlation matrices to determine patterns of shared variation. The high degree of intercharacter differences in sexual size dimorphism indicate that single characters provide poor estimates of size.

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