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Why do birds have to sit on their eggs and for how long?

Sent in by Badger Class at Loose Infant School, Kent

Birds have to sit on their eggs to keep them warm. It's a bit like baking a cake - the warmth from the parent bird makes sure that the chicks inside develop properly. This is called 'incubation'.

To keep the eggs warm, a special warm patch grows on the parent birds' tummies. Some of their feathers drop out so that the warm skin touches the eggs. This is called a 'brood patch'.

Different birds sit on their eggs for different lengths of time. Bigger birds lay bigger eggs which take longer to hatch. Blue tits incubate their eggs for two weeks but swans sit on theirs for nearly six weeks!

Most birds lay one egg a day until the eggs are all laid. Some birds lay up to 15 eggs but others lay only one. But the parent birds do not start to incubate the eggs until they are all laid. In some birds, only the mother sits on the eggs. Others take it in turns to share with the father.

And in a very few bird species - like red-necked phalaropes and dotterels (which are rare wading birds) - only the male incubates the eggs and raises the chicks.

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