Built for flight

Almost every part of bird’s body has been shaped by evolution to meet the demands of flight. Wings and feathers are the most obvious features – they provide the “lift” to overcome gravity. Most birds also have a streamlined shape with weight concentrated in the middle for balance. The bones are riddled with hollow spaces to save on weight, and many are rigidly fused together to reduce the need for heavy joints or unnecessary muscles. The flight muscles are huge and powerful, but they need plenty of oxygen, so birds have special lungs to extract as much oxygen as possible from the air.

Feather light. Feathers are made of fine, lightweight fibres of keratin, the protein that coats bills. Flight feathers have a stiff central shaft, called a quill, with hundreds of side branches called barbs. The barbs bear thousands of tiny branches called barbules, which lock together to form a flat, streamlined surface.

The bare bones. A bird’s skeleton has the same basic plan as a human skeleton, but the details are very different. Birds have only three “fingers” (digits), and these are fused to form a strut supporting the wing. The wing pivots at the shoulder, and the elbow and wrist can bend only horizontally to fold or extend the wing. The tail bones are fused into a stump, and sidebars on the ribs overlap to form a solid cage. An enormous bone called the keel provides an anchor for the powerful flight muscles.

On the wing. The bird’s most important feathers are its flight feathers, found on the wings and tail. Most of the lift required for flight is generated by the primary and secondary flight feathers in the outer part of the wing. There are usually 9-12 of these on each wing. Other parts of the body are covered with small “contour feathers”, which give the bird a streamlined surface, or fluffy down feathers, which keep the bird warm.

How wings work. Wings lift a bird in two main ways. During flapping flight, they push air backwards and down, causing the bird to move forwards and up. Once a bird has picked up speed, the wings catch the wind like sails and create higher pressure underneath, pushing the bird up.

Wingspan. Wings work best when air flows swiftly over them. If the air moves too slowly, turbulent whirlpools develop around the wings and they stop generating lift. The result is a stall: the bird loses its balance and tumbles. Slow-flying birds, such as eagles that soar on thermals, spread out the feathers at the wingtips. This way, each feather acts as a tiny wing, generating extra lift and stabilizing the airflow.

Oxygen supply. Birds’ lungs are far more efficient than ours. When we breathe, air flows in and out of our lungs in two directions. Our lungs don’t empty entirely, so stale air stays behind after each breath. In birds, air circulates through the lungs in one direction only, thanks to a complex arrangement of air sacs around the lungs. Fresh air continually enters the lungs, flushing out stale air and providing a rich supply of oxygen.

Keeping clean. Feathers need a lot of care. The tiny barbules that keep them flat can come unzipped. Birds run their bills through the feathers to zip the barbules back together. Many birds also rub oil from a gland in the rump into the feathers to waterproof them. Some also bathe in puddles or dust to keep the feathers in shape.

Hollow bones. If you are ever picked up the skeleton of a dead bird, you’ll know how light a bird’s bones are. In fact, the feathers weigh up to three times more than the skeleton. The bones are light because they contain a honeycomb of air spaces, criss-crossed by solid to provide strength.

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