Hydrofoil-shaped Bill Draws Prey Closer

photograph of white and pink bird standing on a tree branch

Biological Strategy

Spoonbills

Dimitri Smirnoff

Image: shell game / Flickr / Some rights reserved

Move in/on Liquids

Water is not only the most abundant liquid on earth, but it’s vital to life–so it’s no surprise that the majority of life has evolved to thrive on and under its surface. Moving efficiently in and on this dense and dynamic substance presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag, utilize surface tension, fine tune buoyancy, and take advantage of various types of currents and fluid dynamics. For example, sharks can slide through water by reducing drag due to their streamlined shape and specially shaped features on their skin.

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Capture, Absorb, or Filter Organisms

Many living systems must secure organisms for food. But just as one living system must capture its prey to survive, its prey must escape to survive. This results in capture and avoidance strategies that include trickery, speed, poisons, constructed traps, and more. For example, a carnivorous plant called the pitcher plant has leaves formed into a tube that collect water. Long, slippery hairs within the tube face downward. When insects enter the tube seeking nectar, they lose their footing and slide inside, unable to climb out and escape being eaten and digested by the plant.

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Distribute Liquids

Liquids include water, as well as body fluids such as blood, gastric juices, nutrient-laden liquids, and more. To survive, many living systems must move such liquids within themselves or between locations. Because of their properties, liquids tend to disperse unless they are confined in some way. To address this, living systems have strategies to confine fluids for transport, and to overcome barriers such as gravity, friction, and other forces. Some of these same barriers also provide opportunities. Trees and giraffes face the same challenge: how to move fluids (water and blood, respectively) upward against gravity. But their strategies are quite different. The tree moves water using capillary action and evaporation, possibly due to water’s properties of polarity and adhesion. The giraffe’s tight skin provides pressure to assist in blood circulation. and keep blood from pooling in the legs.

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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 hydrofoil-shaped bill of the spoonbill draws aquatic prey closer by generating swirling flows.

Spoonbills are large wading birds with a distinct bill shape. As the name suggests, the bill is long with a wider rounded end. Spoonbills catch their food in shallow water, where they eat fish, crustaceans, and insects that can be floating in the water or on the muddy bottom. Like many birds, the spoonbill uses its bill to grasp directly at prey that is visible and within reach; however, it also appears to have a strategy for drawing prey that it might not even see closer.

As the spoonbill walks through shallow water, it dips its head down and sweeps its bill from side to side in an arc through the water. The top of the bill is rounded while the bottom is nearly flat. In cross section, this shape is called a hydrofoil (or airfoil if it moves through air–more commonly known as a wing). As a hydrofoil moves through the water, water flowing over the rounded top of the hydrofoil travels faster than water flowing under the flatter bottom. This occurs because the water has to meet at the end of the hydrofoil at the same time (law of conservation of mass). This difference in flow speeds above and below the hydrofoil generates an upward force much like an airplane wing generates lift in air. At the same time, the difference in flow results in areas of swirling water at the tips of the hydrofoil. This swirling water is called a tip vortex, and it’s what researchers believe the spoonbill uses to gather prey.

As the spoonbill’s hydrofoil-shaped bill sweeps through the water, it generates a force and a bill-tip vortex. This vortex can stir up and draw small prey off the bottom of the shallow water or stimulate mobile prey to move in the water column. Once near the spoonbill’s mouth, it can then sense and capture the prey with its bill.

This strategy was contributed by Dimitri Smirnoff and Philip Samuel.

Image: Mwanner / Wikimedia Commons / GFDL - Gnu Free Document License

Roseate Spoonbill - Myakka River State Park.

Image: Greg Lavaty / Katy Prairie Conservancy / Wikimedia Commons / CC BY SA - Creative Commons Attribution + ShareAlike

Image: Chrharshaw / Wikimedia Commons / GFDL - Gnu Free Document License

Image: Frans Vandewalle / CC BY NC - Creative Commons Attribution + Noncommercial

Last Updated June 27, 2017

References

“..A related feature of spoonbills is the shape of the bill: both upper and lower mandibles are dorso-ventrally flattened, the upper mandible being convex in cross section while the lower is tucked in, to result in an almost flat surface. The bill is wide throughout and broadens at the distal end. The hypothesis presented here is that spoonbills use their broad, flattened bills and lateral sweeping to shed a vortex off the tip of the bill that results in hydrodynamic suction on the bottom, which disturbs and moves prey. To achieve this, the bill is used as a hydrofoil. The analysis predicts, and experimental results show that (1) in feeding, the tip of the bill is kept close to the bottom, (2) there is an inverse relationship between bill immersion depth and sweeping speed, and (3) bill sweeping over submerged prey results in the prey being lifted into the water column.” (Weihs and Katzir 1994:649)

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