Sex determination and sexual size-dimorphism in Southern Giant-Petrels (Macronectes giganteus) from Patagonia, Argentina

We used discriminant analysis of a series of external morphometric measurements, together with confirmation by genetic methods, to develop a simple method to determine the sex of fledgling and adult Southern Giant-Petrels (Macronectes giganteus) in colonies in northern Patagonia. We also described morphometric parameters (bill-length, bill-depth, tarsal length, wing-length, and weight) for different classes of age and sex and analysed differences in size between male and female chicks over the growth period. Southern Giant-Petrels showed conspicuous sexual size-dimorphism, with males being significantly larger than females, this difference being apparent within 8 weeks of hatching. In adults, males were 40% heavier than females and consistently larger in all measurements (% sexual size-dimorphism 5–15%). For fledglings, bill-length was the most dimorphic variable (15.2%). The discriminant analysis of four external measurements correctly identified the sex of more than 90% of adults and fledglings. Bill-length alone was sufficient to predict sex of adults and fledglings and can thus serve as a simple and effective tool for sexing Southern Giant-Petrels.