Sex determination of birds is key to understanding various aspects of the demography of a population as it may influence survival, dispersal, recruitment and other life history parameters (Gowaty 1993, Newton 1998), but sex differentiation is difficult in species with sexually monomorphic plumage, even for captured individuals. During the breeding season, the sex of most individuals of many species can be determined either by the presence or extent of a brood patch in females or the cloacal protuberance in males (Ralph et al. 1993). In some species, however, the male can develop an incubation patch, but it does not typically develop as extensively as in females (Skutch 1957, Pyle 1997). Furthermore, during the nonbreeding season these characters are not useful, although sexing birds in the non-breeding season is desirable (e.g., sex-specific migration phenologies, migration routes, and wintering habitat use). Morgan (2005) discussed the possibility of analyzing multi-modal distributions of morphological characters to differentiate sexes. This approach was used by Catry et