If you find a baby bird it's best to return it to its parents. There is no truth to the Old Wives Tale that says the parents will not go near the baby once handled by a human. Birds have no sense of smell - they know each baby by its voice.
If you can see and reach the nest you can put the baby bird back. Make sure the baby is not hurt before returning it to the nest. A healthy baby should feel warm and look alert & active.
It may be necessary to make a substitute nest if you are unable to reach the original nest. You can make a nest with a plastic bucket that is deep enough to hold the baby securely. Just punch holes on the bottom of it for drainage. Put the original nest in the bucket if it has fallen. If the original nest is in tact and hard to reach, then place twigs and dry leaves in the bucket for your replacement nest. Securely fasten or nail the new nest as close to the old one as possible and place the babies inside. The new nest should be at least 5 feet from the ground.
Watch from afar to see if the mother responds. If the adults don't respond within 4 hours, the baby will have to be raised by a wildlife specialist.
Most baby birds need to be fed every 15 minutes for most of the day by their parents. Do not try to give the bird milk or water - just keep him someplace quiet until the new nest is ready or until you can find him help.
If the baby bird is hurt (cold, inactive or bloody), it needs help. Contact your local humane society for a list of wildlife rehabilitators or clinics. The Animal Rescue League of Boston or the MSPCA in Jamaica Plain may be helpful.
Fledglings are birds that are two weeks old. They have feathers and a tail that's about 1/2" long. It is their time to go out and explore. You may see them hopping around on the ground because it takes them a few days to learn how to fly to low branches. It takes them about two weeks to learn how to feed themselves and call to their parents to be fed.
Fledglings should be left alone. We get many calls every spring for "injured" birds that are unable to fly. If the bird looks healthy, active and does not appear to be injured (for example, feather sticking out), the bird is probably a fledgling. Try to keep people and pets away from them and let nature take its course.