A bird's skeleton has a tougher job than a mammal's. It needs to be light enough for flight, but also strong enough to take the strain of flying. To tackle these problems, birds' skeletons have some unique adaptations.
Bird skeletons are surprisingly light for their size. Frigatebirds, which are seen flying over tropical oceans, have a wingspan of over two metres, but a skeleton that is lighter than its feathers. Birds also have lightweight beaks instead of heavy teeth and jawbones. Some other bones are very small, or – like the tail – have disappeared altogether.
A bird’s main limb bones are hollow, with special struts inside to strengthen them. This makes them stronger than a mammal’s without being heavier. Other bones are more rigid than in a mammal’s skeleton. Sideways bones sticking out from the ribs lock them tightly together, and the two collarbones are joined into a single brace – which we call the ‘wishbone’. A rigid skeleton can cope better with the stress of flying.
If you look at the carcase of a roast chicken, you can easily see the huge breastbone, which sticks out like the keel of a boat. This bone is unique to birds. It holds the huge muscles that they need for flying. Most flightless birds such as ostriches and emus have lost this breastbone, since they no longer need it.
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