Leg work

Stilts have very long legs, so they can search for food farther into the water

A bird's leg bones tend to be heavier than a similar-sized mammal's. This is because a bird has just two legs to stand on, so they must be strong enough to support its entire weight, whereas most mammals use four. Birds' legs have adapted to suit many different lifestyles.

Tiptoe

Birds’ knees seem to bend backwards. They don’t: it’s just that we can’t see their real knees. In order to balance on two legs, birds stand on tiptoe and tuck their thighs up close to their body, out of sight. The ‘backwards knee’ that we see is actually the ankle. The part below this, which looks like a shin, is the foot. And the foot – the bit the bird stands on – is just its toes.

Killing and carrying

Eagles kill their prey with their feet. Their legs have to be strong enough to take the impact, and to carry the prey back to the nest. The thick feathers on an eagle’s belly hide the full length of its legs. But you can see on its skeleton that they are surprisingly long.

Wading

Most wading birds use long legs to enter deep water, while keeping their body above the surface to look for food. Stilts have enormously long legs, which help them to find food that other wading birds of their size can’t reach. Long-legged birds often perch on one leg while resting. This helps them to keep warm, since long legs lose heat quickly.

Grounded

Ostriches have even bigger, heavier legs than us. They have no need to save weight, since they are flightless. Instead, they get around by running. An ostrich can run at 70 kilometres per hour – faster than many birds can fly. At full speed, it takes strides of over four metres.

Legless

Swifts have virtually no legs, just tiny toes for clinging to the cliffs and buildings where they nest. They sleep, feed and mate in the air, and never intentionally land on the ground. As they don’t need legs they have adapted not to have them.