Bearded tit Panurus biarmicus, male perched in reed bed, Minsmere RSPB reserve, Suffolk

How birds bodies help them fly

Flying takes a lot of breathing power, and a bird's body is adapted to give it extra puff.

How bird lungs have adapted for flight

Birds have an extra large breathing system, which takes up about one fifth of the space in its body. The average mammal's breathing system only takes up about one twentieth. Birds have an extra strong heart to keep this system working.

Mammals’ lungs

People are mammals. Like all mammals, we breathe air down into our lungs. This air contains oxygen, which makes our muscles work. Our blood carries this oxygen around our body. As it is used up, it is converted to a waste gas called carbon dioxide, which is released by breathing out.

Birds’ lungs

Like mammals, birds also use oxygen and breathe out carbon dioxide. They have special air sacs in addition to their lungs, with hollow bones that allow these gasses to flow around the body more easily. This means that one bird breath goes further and does more work than one mammal breath. It also means that birds have a constant supply of fresh air in their lungs, giving oxygen more chance to enter the blood supply.

Robin, Erithacus rubecula, perched on lichen covered branch in garden. Co. Durham

A big heart

Flapping takes lots of energy.

Birds need to move the blood quickly around their system to keep their flight muscles working. To do this, their hearts are relatively bigger and more powerful heart than a mammal’s.

Small birds’ hearts beat faster than big birds’. A chicken’s resting heartbeat is 245 beats per minute, a house sparrow’s is 460, and a hummingbird’s is 615. During flight, a small bird’s heartbeat rises above 1,000 beats per minute.

House sparrow, Passer domesticus, male, perched on stone in garden. Co. Durham.

How bird skeletons have adapted

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.

Less weight

Bird skeletons are surprisingly light for their size due to having hollow bones. 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 have disappeared altogether, for example in the tail.

More strength

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.

Muscle support

If you look at the carcass 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 as they no longer need it.

Birds, Great Frigate bird. Juvenile in flight. Aride Island, Seychelles