Birds’ bodies are very highly modified to give them the power of flight. Their bones are hollow to keep them light. Their bodies are extremely light too, allowing them to glide and to fly with the minimum of effort. For example, an eagle with a wing span of more than 2 m weighs less than 4 kg. Birds have air sacs linked to their lungs to provide extra oxygen as they flap their wings.
Flying is not just a matter of flapping wings up and down. It is usually a mixture of gliding and powered flight. When the wings are flapped they move in a complicated path, scooping air downwards and backwards. The wings twist so that the air is pushed back in the right direction to provide lift. The wings are twisted again on the forward stroke so that they slide easily through the air and do not slow down the bird’s flight.
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