Sex-specific foraging: The distributional ecology of a polychaete-eating shorebird

Trend analysis showed that in the Dutch Wadden Sea bivalve-feeding birds decreased during the last 30 years, while the worm-feeding birds have increased in that same time. It seems that the decline of bivalve-feeding birds is linked the decrease in available resources, but little is known about the cause of increase in worm-feeding birds. The bar-tailed godwit has been chosen as a model species. This shorebird is a worm specialist and shows high intra- and intersexual differences in size and breeding plumage. The females have a larger body and longer bills than males. This sexual dimorphism has led to different diets: the females mainly forage on deep burying lugworms, whereas the males mainly forage on small shallow burying prey. The difference in occurrence between the sexes at different non-breeding sites was mainly determined by the availability of food (prey burying depth) and not by latitude or flight distance to their Arctic breeding grounds. With increasing conspecific densities, intake rates levelled off in females, but not in males. Both in the field and in the laboratory, lugworms responded to probes by retracting themselves in their burrow, out of reach of the longer-billed females. To predict as to where and how many birds can forage in a certain habitat, measurements were done in the field, as well as in the lab. Although field measurements may yield a consistency test of the distributional patterns in a specific ecological context, they cannot be mechanistically interpreted and are therefore not generally applicable. Using the parameters obtained from the experiment, it was found that in the winter, when lugworms are buried deepest, only the longer-billed females remain in the Waddenzee, while the shorter-billed birds moved to other estuaries, such as the Wash in England. Therefore, not only morphological differences between the sexes, but even within the sexes, play a major role in the distributional ecology of this polychaete feeding shorebird