How do birds fly?

1. To take off, a bird jumps with its strong legs and opens its wings at the same moment. The first big flaps push lots of air down.
2. In slow motion you can see two parts. On the downstroke the wing spreads wide and pushes air down and back. On the upstroke it folds, and the feathers open to let air slip through.
3. A flight feather has a stiff shaft down the middle. Hundreds of thin barbs grow from it, and tiny hooks zip them together into a smooth, light surface.
4. When a bird glides, it holds its wings still. It slowly sinks, swapping height for distance. Long, thin wings like an albatross's glide the farthest.
5. Sunshine heats the ground, and warm air rises in a column called a thermal. Eagles and vultures circle inside it and ride the air upward without flapping.
6. To land, a bird tips its wings up, fans out its tail and pushes its feet forward. The wings and tail act like brakes, so it can touch down gently.

Tap each part. Some of them move when you find them!

• Flight feathers. The long feathers at the end of the wing are called primaries. They overlap to make a smooth wing on the downstroke, then open up on the upstroke.
• Feather zipper. A feather's vane is made of barbs. Tiny hooks on them grab each other, like a zipper or hook-and-loop tape, so air can't leak through.
• Alula (thumb feathers). The alula is a small tuft of feathers on the bird's thumb. When the bird flies slowly, it lifts the alula to keep air flowing smoothly over the wing.
• Hollow bones. Many of a bird's bones are hollow, with thin cross-braces inside. That makes them light but still strong and stiff.
• Flight muscles. A bird's biggest muscles are in its chest. They pull the wings down hard on every downstroke. They attach to a tall ridge on the breastbone called the keel.
• Lungs and air sacs. Birds have air sacs, like small balloons, that pump air through their lungs. Fresh air flows through the lungs when the bird breathes in and when it breathes out.
• Tail. The tail fans out and tilts to steer, balance and brake, a bit like a rudder and flaps together.
• Light beak. A beak is light, with no heavy teeth or jaw muscles up front. A muscly stomach called the gizzard grinds the food instead.

Choose a way to fly, a wing shape and the weather. Watch the bird's height and energy. Which wings glide best?

Experiment: Which wings let a gliding bird stay up the longest in calm air?

1. Choose Glide and calm air.
2. Try each wing shape and watch how fast the bird sinks.
3. Now choose Soar and warm rising air. What changes?

What we found: Long, thin wings glide the farthest because they make lots of lift with little drag. Short, round wings sink faster, but they are great for quick takeoffs and dodging through trees. And no wing can soar upward without rising air!

Turn on JavaScript to try it yourself.

Pick a wing shape and fly to the nest. You only have 10 flaps, so use the warm rising air to climb.

A bird's wing is curved like an airplane wing, so air flowing past it gets pushed downward. When the bird pushes air down, the air pushes the bird up. On every downstroke, the wingtip feathers also push air backward, which moves the bird forward.

Flapping is hard work, so many birds save energy by gliding. A gliding bird holds its wings still and slowly sinks, like sliding down a long, gentle hill made of air. Birds with long, thin wings, like albatrosses, glide the farthest.

Soaring is the cleverest trick. When sunshine warms the ground, bubbles of warm air rise up. Eagles, vultures and storks find these thermals and circle inside them, rising higher and higher without flapping. Birds also soar on wind that is pushed up over hills and cliffs.

Every part of a bird helps it fly: hollow bones and a toothless beak keep it light, big chest muscles power the wings, air sacs bring in plenty of air to breathe, and feathers zip together into strong, light wings.

Key words

Downstroke

The powerful downward flap that makes most of a bird's lift and thrust.

Thermal

A column of warm air that rises from ground heated by the sun.

Glide ratio

How far a bird travels forward for each bit of height it loses when gliding.

Sink rate

How fast a gliding bird loses height, in meters per second.

Primaries

The long flight feathers on the outer part of the wing.

Wing loading

How much weight each bit of wing has to hold up.

Check your thinking

1. On which stroke does a bird make most of its lift and thrust?

   Show the answer

   The downstroke. The downstroke pushes air down and back, which pushes the bird up and forward.

2. What do eagles and vultures ride to rise without flapping?

   Show the answer

   Thermals of rising warm air. Warm air rises in thermals. Circling inside one lifts the bird higher for free.

3. Which bird wings are best for gliding a very long way?

   Show the answer

   Long and thin, like an albatross. Long, thin wings make lots of lift with little drag, so the bird sinks very slowly.

4. How do hollow bones help a bird fly?

   Show the answer

   They are light but still strong. Thin cross-braces inside keep the hollow bones strong while saving weight.

5. A gliding bird enters a thermal rising at 2 m/s. While circling, it sinks through the air at 1.2 m/s. What happens?

   Show the answer

   It climbs at about 0.8 m/s. Its speed over the ground is the updraft minus its sink rate: 2 − 1.2 = 0.8 m/s upward.

6. What does the alula (thumb feather) do?

   Show the answer

   It helps the bird fly slowly without stalling. Lifting the alula keeps air flowing smoothly over the wing when the bird flies slowly.