If the bird is a hatchling, that is it does not have feathers or looks totally helpless, you can look for the nest from where it came and return it there. The next best thing is to secure a basket in a bush or tree and place the bird in the basket with the hope, albeit slim, that the parent will find it and care for it.
Most birds do not have a keen sense of smell, so handling a baby bird will in no way deter a parent from caring for it. Remember, even if you had 15 hours a day to feed and care for the bird, it would not learn the skills it needs to survive in the wild if it were raised by humans. If you don’t know where the nest is, or the bird is injured, keep the bird warm and safe until you make arrangements with a licensed wildlife rehabilitator for its continued care.
Do not attempt to feed it or give it water unless instructed to do so by a rehabilitator. A list of licensed wildlife rehabilitators can be obtained from the Massachusetts Division of Fisheries and Wildlife Web site at www.masswildlife.org.
If the bird is a fledgling, that is it has feathers and it is just flightless, and it is not visibly injured, it is best to leave it alone. Its parents are likely nearby and often are very sneaky about feeding the offspring when they won’t be detected. If there is imminent danger, such as a cat, try to remove the danger, not the bird. Put the cat inside where it belongs or if it belongs to a neighbor, ask the neighbor to remove the cat. Or try to shoo the cat away with water. Place the bird on a branch up out of the way if that will improve its chances.