Be sure the bird is in need of help as sometimes they leave the nest before they can fly well and already have feathers. The parents may be nearby or will return soon. Hatschlings are bald or may have tufts of feathers and are very small. if the parents don't return within a half hour, try to find the nest and carefully place the bird back inside. Or if you can get a small basket, line it with grass, place the bird in it, cover with a piece of paper and place it as close to the tree where you found it as possible. You can even nail the basket to the tree to avoid predators. The parents will be able to hear the baby when they return. You can also call wildlife rescue to ask for help.