Evidence of a new feeding mode in western Sandpiper (Calidris mauri) and dunlin (Calidris alpina) based on bill and tongue morphology and ultrastructure

Sandpipers of the genus Calidris tend to have similar body shapes and use narrow, tubular bills to feed on invertebrates in aquatic habitats over an extensive migratory range. Highly plastic foraging behaviours have been displayed but the associations between diet and feeding mechanics are less well understood. Here, scanning electron microscopy was used to relate the ultrastructure of the bills and tongues of two sympatric congeners, western sandpiper (C. mauri) and dunlin (C. alpina), to function and elucidate the sensory and mechanical basis to feeding. The morphology of the relatively larger, more robust bill of the dunlin suggests specialization for probing although both species are known to feed on epifaunal and infaunal prey. In general, external and internal bill features appear similar for the congeners and the microstructure of taste bud and salivary gland complexes in their mouth cavities correspond to descriptions for other birds. However, the tongues of the two species are remarkable for their distinctive micro-structural details as well as copious mucus and extraneous material. In particular, each species has unique tongue papillae, and the keratinized lateral spines along the edges and at the tips of the western sandpiper tongue are markedly longer and denser than for those of the dunlin. Based on the unique features of the tongues and associated observations, inter-species differences in foraging performance can be inferred. Further, evidence suggests that the western sandpiper, and, to a lesser extent, the dunlin, deposit feed on surficial biofilm materials, as opposed to incidentally imbibing such materials while targeting epifaunal prey. Commensurate with the evidence, a novel functional and mechanistic case for unfiltered biofilm grazing in birds is proposed. [PUBLICATION ABSTRACT]