Morphometric and molecular sexing and sex ratio of Dunlin Calidris alpina pacifica at the southern extreme of their winter distribution

The Dunlin Calidris alpina has monomorphic plumage outside the breeding season and only slight size dimorphism between sexes, with considerable phenotypic variation. Although sexing models (Discriminant Function analysis combined with genetic analysis) have been generated for several subspecies, the phenotypic variability of Dunlin within and between subspecies means that multiple models are required. By applying morphometric and molecular sexing, we assessed the sex ratio of pacifica Dunlin in Guerrero Negro (Mexico), close to the southernmost extreme of its non-breeding distribution. A total of 760 birds were captured and measured, and blood samples were collected from 126 individuals for molecular sexing. Culmen length was the variable with the most evident dimorphism. In 81% of the individuals that had been sexed molecularly, a normal distribution curve for culmen length of each sex correctly allocated sex. Additionally, a step-by-step discriminant equation with four variables (head, culmen, tarsus and wing chord) correctly identified sex in 87% of individuals. When applying the equation to the full dataset, there were no significant differences in sex ratio in autumn and winter. By contrast, in spring the sex ratio was heavily male-biased. Therefore, we suggest that females depart earlier than males and may use a higher number of stopovers, using an energy minimization strategy.