Latitudinal clines in bill length and sex ratio in a migratory shorebird: a case of resource partitioning?

Sexual segregation outside the mating season is common in vertebrates. This has been shown to arise through resource partitioning in a number of taxa, but never in avian migrants. Western sandpipers (Calidris mauri) are migratory shorebirds that mainly breed in Alaska and overwinter along the coast in western and eastern North America down into South America. Females are slightly larger than males but have substantially longer bills. They migrate further south, resulting in a latitudinal bias in sex ratio. Resource partitioning could contribute to this pattern if (1) males and females forage on invertebrates buried at different sediment depths, and if (2) average prey burying depth changes with latitude. In accordance with the hypothesis, it was shown that female western sandpipers used a probing foraging mode more than males. Comparison of individuals overwintering in California, Mexico and Panama revealed that length of tarsus did not change among sites, while bill length increased in both males and females towards the south. Bill length residuals, corrected for length of tarsus, were also larger at a southern site. Between and within sexes, individuals with longer bills are thought to be favoured at southern latitudes because of a postulated general increase in invertebrate burying depth with higher ambient temperatures at lower latitudes. The implications of these findings for the evolution of sexual bill size dimorphism in shorebirds are discussed.