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White-crested Elaenia (Elaenia albiceps chilensis) is a Neotropical austral migrant that is sexually monomorphic in plumage. We first determined whether the brood patch and cloacal protuberance are good indicators of an individual’s sex by comparing these characters with individuals sexed using molecular techniques. Second, at several localities encompassing a 1500 km latitudinal gradient across the breeding range, we evaluated whether morphological measurements can be used for sex determination, using discriminant analysis and molecular sexing as dependent variables. Finally, the effectiveness of the discriminant analysis was assessed by a Jackknifed validation, and by a cross-validation process through the classification of a new sample. Sexing using genetic techniques and by cloacal protuberance size and presence of the brood patch produced the same results. We did not find differences in body measurements among study sites. In all localities, males were significantly larger than females in wing and tail lengths, whereas other variables did not exhibit differences between sexes. The best classification of sex by discriminant functions was obtained by including wing and tail length as discriminatory variables. The discriminant function correctly determined the sex of 86 % of all samples, with correct identification of 90 % of males and 75 % of females. The Jackknifed validation and cross-validation of a new sample resulted in similar sex classifications as those produced using the discriminant function. Discriminant analysis thus represented a simple and cost-efficient way to determine the sex of White-crested Elaenias for field ornithologists.

Sex determination by morphometry of adult White-crested Elaenia (Elaenia albiceps chilensis)