A nestling is a baby bird that can't fly yet and tends to hang out in its — yes, you guessed it — nest. Nestlings have to be fed and cared for by their parents.
From the moment a baby bird hatches from its egg, it is a nestling — and it remains a nestling until it learns to fly, when it officially becomes a fledgling. Some people fondly refer to young children as nestlings too. This noun adds the diminutive ending -ling (as in gosling for baby goose) to nest.