Most birds leave their nests when they can feed themselves. This usually occurs when the birds turn into fledglings and are two weeks old. Depending on the species, some birds may wean for another week or two or until they are able to fly and eat on their own.
The fledglings are called juveniles when they grow as big as an adult bird and can fly on their own. While some juveniles may still beg for food from their parents, most parents eventually leave and let the youngsters feed for themselves and fatten up before the migratory flight. When a juvenile bird leaves the nest, it is unlikely to return to it.