"Many baby birds leave the nest before they are able to fly. The reason they do this is varied. It could be that the nest became too small to accommodate all the babies (they?ve been growing at a rapid speed) or because parasites have invaded the nest, or because they sense they have a better chance against predators being out of the nest, but mostly because the parents have coaxed them, one-by-one, out of the nest because they knew instinctively it was time for their babies to take their first flight!
The parents have not abandoned them, they are close by, watching and caring for these babies. They bring food to them throughout the day and within a short period of time (days) the babies are flying, not gracefully, but flying short distances and then they follow their parents who will show them the best sources of food and water.
The best thing to do is to leave it there, if you have picked it up, bring it back to the exact area you found it, place it in or under a bush. The parents have, most likely, been frantically looking and calling for this lost baby. You can wait and watch for a few hours to make sure the baby bird is OK, but do this from as far away as possible so you don?t frighten the parents who are waiting for a safe time to approach the baby bird. If after watching from a distance for several hours you cannot see the bird?s parents, follow the previous instructions and call your nearest Wildlife Rehabilitator. (Links listed above) (Exceptions: You see a baby bird is in an obviously dangerous situation like sitting in the middle of the road. Pick it up and place it in a nearby bush where parents will still find it easily.)"