The chicks went a few weeks ago, so why is there a pigeon still nesting in our garden?

I saw the chicks a few weeks ago and they were nearly fully feathered (Don't worry I didn't disturb the nest to see them. The nest is in a position so you can see it easily). Now the chicks I assume have gone, but the female pigeon is still sitting in her nest as if she's incubating eggs...Why?