Most of the questions along this line refer to songbirds and doves. The young are hatched naked, blind, and completely helpless. They will not develop the ability to maintain a constant body temperature until they are a week or more old. Perhaps it is this very helplessness that makes so many people want to come to the rescue when they find an abandoned nest or a nestling on the ground.
People often assume that a nest or nestling is abandoned if the parents are out of sight for more than a few minutes. Not necessarily so. In the case of an intact nest, back off and 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, 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. Contrary to popular belief, most birds have almost no sense of smell. The parents will not abandon a nest or a chick simply because it has been touched by a human. They may very well abandon, however, if you decide to revisit the nest frequently to 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. Consider using the experience to teach your kids some of the harsher realities of Mother Nature.
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. Unless you can do so, do the nestlings a favor and leave them alone. Even though they will almost certainly die, death will come far more rapidly and humanely to a young bird than to one that has been hand-reared to an older age before its foster parent loses interest.
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. No matter how much care you are willing to devote to them, you really can't give them the tools and skills that they need to survive.
And it applies even more to an abandoned clutch. Heroic efforts taken to rear a clutch of eggs from the embryo to fledgling stage may make you feel good, but is it fair to the birds? Take solace in the fact that the adults will renest somewhere else, and leave the eggs where you found them.
Be advised that all native migratory birds are protected under the Migratory Bird Treaty Act, and that captive rearing without proper authorization is a violation of federal law. In many cases it is also a violation of state law to hold migratory species in captivity. Additional laws, such as the Raptor Act, which covers hawks and owls, apply to specific groups of birds. Protect yourself by contacting your state natural resource agency and the U.S. Fish and Wildlife Service before you take nestlings into possession. At the request of the Fish and Wildlife Service, Northern Prairie Wildlife Research Center will not disseminate information on hand rearing birds.
Good luck, Dave Fellows