Beak Pulls Up Fluid Droplets

Biological Strategy

Red-necked phalarope

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Capture, Absorb, or Filter Liquids

The most common liquid used by living systems is water, which they require to survive. But there are many other liquids that provide nourishment, play a role in defense mechanisms, or serve other purposes. Water varies in its availability; it is sometimes plentiful and sometimes very scarce or only available as fog or mist. To minimize the energy required to capture, absorb, or filter liquids, living systems have strategies that take advantage of the unique properties of the given liquid. For example, water moves from a gaseous to liquid state when it encounters a surface colder than the air. Plants in forests that experience fog and clouds more than rain have strategies that condense liquid water from moist air.

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Birds

Class Aves (“bird”): Eagles, hawks, sparrows, parrots

Birds are evolutionary engineering marvels. They are descended from dinosaurs, but are far from our idea of heavy, scaly reptiles. Of the specific adaptions that set them apart, most notable is flight—although some mammals can fly, birds take the prize for abundance in the skies. Many birds have hollow, lightweight skeletons and specially-designed wings to help them stay aloft. They also have feathers made of keratin that help them stay warm, attract mates, and improve navigation and aerodynamics in flight. In contrast to their dinosaur ancestors, they lack true teeth and have replaced them with specialized beaks and bills.

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The beak of the red-necked phalarope pulls up fluid droplets via capillary action created by repeatedly opening and closing.

The red-necked phalarope feeds by pulling up suspended particles from the water below through its beak. Since this movement is against gravity, the beak must generate a force to pull the fluid. The bird achieves this by opening and closing its beak slightly in a repeated ratcheting, or tweezering, motion.

This opening and closing motion changes the fluid’s angle of curvature in contact with the beak, generating pressure from surface tension that pulls the fluid. As the beak closes, the leading edge of water moves toward the mouth; as the beak opens, the trailing edge of water recedes toward the mouth. Because of the pressure difference, the fluid’s net movement is toward the mouth. This mechanism is known as the “capillary ratchet.”

See the capillary ratchet in motion in the second half of this video.

This strategy was contributed by Rachel Major.

Last Updated August 23, 2016

References

Surface Tension Transport of Prety by Feeding Shorebirds: The Capillary Ratchet

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