The bill of the sandpiper draws food into its mouth using water’s surface tension.

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The western sandpiper (Calidris mauri) is a shorebird that spends its winters on the coasts of the US. Probing in mud and sand with its long bill, the sandpiper picks up small prey like marine worms, crustaceans, and insects. It can also feed on tiny plankton floating in the water, but these prey are too small to probe or peck at individually. To eat plankton, the western sandpiper uses a strategy called “surface tension transport.”

Water has a surface tension that results from attractive forces between water molecules. At an air-water interface, this surface tension tends to keep the interface as small as possible. Spreading water out and creating more interface takes energy. Water molecules are also attracted to keratin, the protein material that makes up the sandpiper’s bill. As it wades along the shore, the sandpiper dips the tip of its bill into the water and grasps at plankton like a pair of tweezers. It then lifts its bill out of the water and opens the bill slightly. The plankton-rich drop of water at the bill tip sticks to the upper and lower bill and stretches apart, creating an air-water interface. As the sandpiper continues to open its bill, the water drop continues to stretch. Water’s surface tension then reduces the stretched interface area by moving the drop up towards the sandpiper’s mouth. This delivers tiny plankton in the water drop to the throat where they can be swallowed.

To see what the stretched water drop looks like in another shorebird, check out this site about surface tension transport feeding.

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“Instead of filtering, as do most planktivores, these small birds ‘tweezer’ prey. But that puts the prey at the tip of the bill, not the pharynx. Gaping the bill slightly, though, creates an [air-water] interface, as in fig. 5.8a). The water then does its part – surface tension reduces the area of interface by making the droplet of water move up and back from the bill’s tip.” (Vogel 2003:107)

Comparative Biomechanics: Life's Physical World, Second EditionJune 17, 2013
Vogel S.

“Surface tension prey transport is a feeding mechanism employing the surface tension of water surrounding prey to transport prey from bill tip to mouth…Laboratory investigations of the feeding mechanics of Wilson’s Phalarope Phalaropus tricolor, Western Sandpiper Calidris mauri and Least Sandpiper Calidris minutilla demonstrated that all three use surface tension transport of prey when feeding in water.” (Rubega 1997:488)

Journal article
Surface tension prey transport in shorebirds: how widespread is it?IbisJanuary 1, 1997
Rubega MA

“…STT [surface-tension transport] is a common feeding mechanism in small or medium-sized shorebird species that feed on small prey items in shallow water. Birds using STT transported ≤3.6× faster than the theoretical value predicted by a previous model and can achieve high intake rates when foraging on high densities of available small prey items.” (Estrella et al. 2007:1244)

Journal article
Small-prey profitability: field analysis of shorebirds’ use of surface tension of water to transport preyThe AukJanuary 1, 2007
Estrella SM; Masero JA; Pérez-Hurtado A

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Western Semipalmated SandpiperCalidris mauriSpecies

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