Sexual size dimorphism and sex determination by morphometric measurements in the Coscoroba Swan

Abstract

The accuracy of morphological sexing and the occurrence of sexual dimorphism were analyzed in mature and immature Coscoroba Swans (Coscoroba coscoroba, Anatidae) near the Estação Ecológica do Taim, southern Brazil. On the basis of weight and 10 linear measurements of external morphology, multivariate analysis of vari- ance showed that males were consistently larger than females (sex confirmed via genetic markers) and mature birds were consistently larger than immatures. Overall, 38% of immatures and 14% of mature birds were sexed incorrectly by cloacal examination when compared to genetic data. Therefore, we performed a discriminant function analysis of different age classes based on morphometric measurements. Mature birds were sexed with 96% accuracy using head and tarsus lengths as predictor variables, whereas immatures were sexed with 90% accuracy based on head and forearm lengths. Method validation conducted with data for additional mature sampled in a different year showed that the use of head length alone was as accurate for sexing (92% correct classification) than discriminant functions based on two characteristics (91%).