How Does a Bird Stay Up?

Step 1 of 5

A wing pushes air downwards.

A wing is curved on top and flatter underneath. Air flowing over it gets bent downwards as it leaves, and pushing air down is what pushes the bird up.

Step 2 of 5

The bones are hollow.

A bird's bones are not solid. Many are hollow tubes with little struts crossing inside, which keeps them stiff while weighing far less — the same trick as a scaffolding pole.

Step 3 of 5

The muscles are all in the chest.

Flight muscles can be a quarter of a bird's weight, and they are bolted to a deep keel on the breastbone. Keeping the weight low and central is what stops it tumbling in the air.

Step 4 of 5

A feather is a zip.

Each feather's branches carry tiny hooks that clip onto the branch beside them. That turns a loose fan into a solid, airtight sheet — and a bird re-zips them by drawing them through its beak.

Step 5 of 5

Not every bird flies.

An ostrich, a penguin and a kiwi all gave it up. If food is on the ground or in the sea, and nothing is hunting you, flight costs more than it is worth — so it gets traded away.

Birds breathe with air sacs as well as lungs, so fresh air flows through in one direction — which is why they can fly over mountains that would leave you gasping.