Functional association of bill morphology and foraging behaviour in calidrid sandpipers

Foraging behaviour in birds co-varies with bill morphology. Shorebirds exhibit pronounced inter- and intra-specific variation in bill length and shape as well as in foraging behaviour. Pecking, or feeding on epifaunal intertidal invertebrates, is associated with a straight bill, while probing, feeding on infaunal prey, is facilitated by bill curvature. Here, we used high resolution microscopy to study gross bill morphology of Western Sandpipers (Calidris mauri). We showed that bills of males and females differed with regard to length but not curvature or depth, despite clear differences in foraging behaviour between the sexes. Detection of infaunal prey can be facilitated by the presence of Herbst corpuscles. These mechano-receptors are located in 'sensory pits' under the keratin layer of the bill and are able to sense pressure gradients. They are postulated to be common among calidrid sandpipers, but comparative data are lacking. Using high resolution microscopy, we measured number and size of sensory pits in Western Sandpipers, Least Sandpipers (Calidris minutilla) and Dunlin (Calidris alpina). The implications of these findings to foraging adaptations and non-breeding site choice are discussed.