The embryo grows inside the egg by feeding on the yolk. Then when it is big enough it hatches by pecking through the shell with its beak. Most birds lay their eggs in a nest. Magpies, jays and many other birds build nests in trees from twigs. 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.