Warblers build nests in grass. Sand martins dig out burrows in sandy banks. Robins nest in any hole they can find, whether it is a crack in a wall or an old teapot. Most newly-hatched birds are naked and helpless, but soon grow a covering of soft feathers called down. They stay in the nest, and are fed by their parents until they are ready to learn to fly or are strong enough to fend for themselves.
Most young birds grow very quickly indeed.
A young robin weighs about two grams when it hatches, but within about eleven days it is more or less fully grown. Beaks and bills Instead of a mouth, birds have a hard beak or bill. Beaks are adapted for many tasks - some for building nests, some for breaking nuts, some for catching insects, some for extracting worms from deep mud. The shape depends largely on the kind of food the bird eats. Birds of prey have hooked bills for tearing flesh.