(noun) A young, recently hatched bird that has not yet grown any flight feathers and is unable to fly. They may lack feathers entirely or could be covered with a light down, depending on the species. Nestling birds are entirely dependent on their parents for food and protection, though the length of that dependency varies for each bird species from just a few days to several weeks. One or both parents may feed the young birds and stay with them to help regulate their body temperature until they acquire thicker plumage.
As birds mature and begin to grow their adult flight feathers, they are no longer called nestlings and instead are referred to as fledglings.