If you come across an injured bird, fight your instincts to handle it unless absolutely necessary. Help a baby bird that has fallen out of a nest or a fledgling that is hopping around by keeping an eye out for a parent, who is probably nearby. Letting nature takes its course is usually the best course of action.
If the bird is a nestling -- featherless and covered in down -- attempt to return it to its nest and let the parents take over. If you cannot locate or reach the nest, make one out of a small plastic tub lined with paper towels and carefully place the bird inside. Keep the nest near where you found the bird. Watch to see that the parents rescue the nestling.
If the bird is a baby with feathers and is hopping around on the ground, do not handle it. It is normal for fledglings, who have feathers but have not yet learned to fly, to spend time moving around on the ground. Very likely, the parents are nearby.
Do not touch a bird that has flown into your window and is sitting on the ground. It may just be stunned and will fly away when it regains it senses. Call an animal control agency if the bird is obviously injured or dead.
Call the Audubon Society or Humane Society to determine how to handle an injured bird.