People often assume that a nest or nestling is abandoned if the parents are out of sight for more than a few minutes. This is not necessarily the case. In the case of an intact nest, watch unobtrusively for several hours to determine that it really is abandoned. Any partially feathered youngster that is capable of perching is probably in the early stages of fledging, and chances are that the parents will care for it if you leave it alone. On the other hand, you can safely assume that a naked, immobile nestling found on the ground under a nest tree is abandoned.
The best thing you can do with a nestling that has fallen out of the nest is simply to put it back in the nest. Most birds have almost no sense of smell and the parents will not abandon a nest or a chick simply because it has been touched by a human. However, the parents may abandon the nest and young if you decide to revisit the nest frequently check on “your” nestling.
A very practical alternative, especially if the nest is beyond easy reach, is to take a hands off approach and do nothing. Climbing into a nest tree not only risks injury to yourself, but it also risks damage to the nest or to the foliage that hides the nest from predators. Nestling mortality under natural conditions is very high, and our common songbirds have evolved reproductive strategies that compensate for such losses.
This also applies to an abandoned nest. All too often, people take the nestlings home and attempt to hand rear them. Though well intentioned, few people are willing and able to provide dawn-to-dusk care for up to two weeks, followed by another two weeks or so of post-fledgling care.
Even if you are willing to make the commitment, consider that, in the long run, hand-reared birds have a low probability of surviving in the wild. Parents teach their offspring to feed and to avoid danger. They also socialize the offspring.
All native migratory birds are protected under the Migratory Bird Treaty Act, and that captive rearing without authorization is a violation of federal law and, in many cases, also a violation of state law. Contacting your state natural resource agency and the U.S. Fish and Wildlife Service before you take nestlings into possession. Your local animal control office or zoological society may also be able to provide you with the names of licensed wildlife rehabilitators in your area.