Sexual size dimorphism and assortative mating in the short- tailed shearwater puffinus tenuirostris

Abstract

To improve methods for sexing live birds in field studies, we assessed sexual size dimorphism in the Short-tailed Shearwater Puffinus tenuirostris and produced a sex-discriminating function. Despite a degree of overlap in body size, males were significantly larger than females. A stepwise discriminant function analysis of five morphometric characters indicated that bill depth and head length were the most dimorphic characters, and the resultant sex model correctly discriminated 92.0% of known males (23 of 25), and 92.3% of known females (24 of 26). The model was validated by applying it to an additional group of birds whose sex was assumed, based on their pairing with known-sex individuals. Of the assumed females, 93% were correctly classified (n = 15), as were 96% of males (n = 15). Application of the sex model to another breeding colony reduced its performance to 70%–82% accuracy because of the existence of significant geographic variation in body size in this species. For individuals in which certainty was low (i.e. when small males are confused with large females), sexing could be improved by measuring the body size of the breeding partner. This improvement was a result of significant positive assortative mating with respect to bill depth and a body size index. This sex model provides a quick and easy means of sexing in instances in which molecular methods and other techniques are not feasible.