Bending Prevents Damage

white and brown owl flying during daytime

Biological Strategy

Birds

Eleanor Banwell

Image: Danny Moore / Pixabay / Free non-commercial use

Manage Turbulence

A turbulent force occurs when air or water creates a chaotic or irregular motion. The source can be such things as wind, waves, and eddies caused by obstructions to air or water flow (such as that created by a rock in a stream). Because the force is irregular, it acts in unpredictable ways on multiple parts of a living system at any given time, decreasing the living system’s efficiency. Strategies used to manage turbulence include dampening the amount of turbulence, having flexibility to handle sudden changes, and making quick adjustments. An example is the mucus on aquatic organisms, such as barracuda sharks, that can reduce turbulent friction of seawater by 66%. In doing so, it decreases drag and increases the sharks’ swimming efficiency.

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Manage Shear

The effect of shear stress on a living system is parallel internal surfaces sliding past each other. Slippage occurs in parallel with the force. Think about holding two wooden boards on top of each other and sliding one to the right and the other to the left. This may be easy until you add glue, which increases their shear strength and makes them harder or impossible to slide. Shear can occur in solids, liquids, and gases. Living systems must increase their shear strength to overcome these types of forces. For example, darkling beetles lock their wings together in flight to prevent lateral movement by using many small hairs on each wing. These hairs interlock to provide shear strength, just as two hair brushes put together would be difficult to slide past each other.

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Manage Compression

When a living system is under compression, there is a force pushing on it, like a chair with a person sitting on it. When evenly applied to all sides of a living system, compression results in decreased volume. When applied on two sides, it results in deformation, such as when pushing on two sides of a balloon. This deformation can be temporary or permanent. Because living systems must retain their most efficient form, they must ensure that any deformation is temporary. Managing compression also provides an opportunity to lessen the effects of other forces. Living systems have strategies to help prevent compression or recover from it, while maintaining function. For example, African elephant adults weigh from 4,700 to 6,048 kilograms. Because they must hold all of that weight on their four feet, the tissues of their feet have features that enable compression to absorb and distribute forces.

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Manage Tension

When a living system is under tension, it means there is a force pulling on it, like a person pulling on a rope tied to a horse. When applied to a living system, unless the system is completely rigid, the result is that it gets stretched. If stretching exceeds the strength of the living system’s material, it can damage it. Living systems manage tension using materials that are flexible and stretchable enough to survive most tension that occurs in their environment. The ocean’s intertidal zone offers a good example. The waves and incoming and outgoing tides put tension on soft-bodied organisms. Mussels resist tension with flexible threads that hold them onto rocks; in contrast, large algae have stretchy fronds.

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Move in/Through Gases

Living systems must move through gases (which are less dense than liquids and solids) such as those in the earth’s atmosphere. The greatest challenge of moving in gases is that because the living system is heavier than the gas, it must overcome the force of gravity. Moving efficiently in this light medium presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag and increase lift so that they can stay aloft and take advantage of variable currents. Additionally, they must overcome gravity when moving from a liquid or solid into the air. The fairyfly, the smallest known insect, is a tiny wasp that must move through the air. To the wasp, air feels like a heavy liquid and to move through it, it uses special feathery oars rather than wings.

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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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Flight feathers of birds avoid damage by bending out of the way.

Introduction

Birds have mastered the art of flight, and their feathers are a marvel of natural engineering. Unlike solid airplane wings, bird flight feathers must be both strong and lightweight to support lift and withstand the forces of air resistance and turbulence. A key feature that prevents damage and ensures longevity is their ability to bend and flex in response to excessive forces. This allows birds to maintain efficient flight while reducing the risk of feather breakage.

The Strategy

Flight feathers are unique structures that are credited with being the primary evolutionary development that enabled birds to fly. They must be able to withstand the large forces that occur during flight and they must be lightweight. Unlike airplane wings, which are solid, bird flight feathers are formed from a web of barbs projecting from a central stiff vein. This design balances the large surface area needed for lift with strength and extreme lightness.

Feather barbs are thin and flexible and have a number of features to protect them from permanent damage. Barbs are linked with a series of grooves and hooks that zipper up like Velcro, enabling them to detach when forces become too great and to be easily repaired later. Barbs are ribbon shaped, meaning they are stiff in one direction and flexible in the other. They are stiff in the direction under which they are loaded during flight, but very flexible in the other direction. This works because the hook and groove mesh holds them in place and counteracts their side-to-side flexibility. At the same time, if forces do become too large and the barbs are at risk of permanent damage, they are still able to twist, presenting their flat flexible side to the force, bending and spilling the load. In this way, they flex out of the way when necessary, reassuming their prior position later and zippering back up again, as good as new.

Image: Nathan / Public Domain - No restrictions

Image: EvgeniT / Public Domain - No restrictions

The Potential

The design of bird feathers offers inspiration for a wide range of human applications. In aerospace engineering, flexible, self-healing materials could improve aircraft wings, making them more resilient to turbulence and stress while reducing maintenance needs. Additionally, architects and engineers designing wind-resistant structures could take inspiration from the way feathers flex to dissipate force, creating architectural features that can temporartily deform to avoid breaking in extreme weather conditions.

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Nature’s Ingenious Velcro: Interlocking Feather Hooks in Birds

Birds

Tiny hooks allow the filaments of a feather to hold smoothly together or be easily separated.

Last Updated July 23, 2019

References

Journal article

A lightweight, biological structure with tailored stiffness: The feather vane

Acta. Biomater | 01/01/2016 | Sullivan TN, Pissarenko A, Herrera SA, Kisailus D, Lubarda VA, Meyers MA

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