How do birds fly?

Birds are built for flight from the inside out. Their bones are hollow and light, their chest muscles are huge, and their feathers are strong but almost weightless.

A bird’s wing is curved: rounded on top and flatter underneath. As the wing moves forward, air travelling over the curved top has further to go, so it moves faster and its pressure drops. The slower air underneath pushes up more than the air on top pushes down. This upward push is called lift.

Flapping does two jobs: the downstroke pushes air back and down to move the bird forward, and the shape of the wing turns that forward motion into lift.

Big birds like eagles save energy by gliding on warm rising air called thermals. Aeroplane wings copy the same curved shape — which is why they look like a bird’s wing from the side.