First the birds are hatched from their eggs, then the young are taken care of by their parents until they are old enough to leave the nest. Then after the parents are finished laying their eggs for the year, the parents leave the nest. All that it was used for was to lay the eggs and to raise the young birds until they left the nest. When there are no more young birds the parents also leave the nest.
You may be used to birds building nests but depending on the type of bird that is not the way that it always occurs. Some birds build their nests, yes, but there are also birds which take over another birds empty nest. Other birds use their own nests from the previous year after inspecting it. Still other birds do not use nests but use gutters, cavities, holes etc.
|Green Woodpecker in tree nest.|
|Female House Sparrow building nest in gutter.|