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Royal Society for
the Protection of Birds

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Home Adaptations Skeletal System Hollow Bones
Whooping Crane
Whooping Crane
(Courtesy NEBRASKAland Magazine/NGPC)

Suppose you're a whooping crane migrating from the Gulf of Mexico north to Canada. That will be a long flight! The more body weight you have, the harder your trip will be.

A whooping crane is one big bird. How does it manage to fly so far with all that "baggage" on board?

Actually, a whooping crane is large but lightweight. Like all birds, whooping cranes have hollow bones that make their bodies lighter.

With hollow bones a bird can fly very long distances without getting worn out from carrying its own weight.

Hollow bones look like other bones, with the usual hard exterior you'd expect a bone to have. But instead of being filled with marrow as your bones are, a hollow bone has an air cavity inside.

Top, bird bone; bottom, human bone

Top, bird bone; bottom, human bone

The hollow part of a bird bone isn't wasted space. In some bones, the hollow cavities contain extensions of the air sacs from the lungs. These air sacs help the bird to get the oxygen it needs to fly quickly and easily.

You might think these bones are fragile, like empty egg shells, but birds can't afford to have bones that break easily. The hollow bones are supported by internal struts – structures inside that help brace the bone so that it can withstand longitudinal pressure (pressure along its length).

The struts within a bird's bones are much like the struts that support the hollow wings of airplanes. They give the bone added strength, so it can withstand the rigors of take offs, flights, and landings – not to mention all the other activities in a bird's life.

