There is just something about babies, whether animal or human, that makes you want to nurture and care for them. Perhaps it is their utter helplessness, or their plaintive cries for food. Regardless, many people with the best of intentions attempt to "help" young birds they encounter on the ground. It can be difficult to determine when to intervene, and what to do if a bird actually does need help.
First, it is important to determine the age and level of independence of the bird. Just as not all mammals are born with the same level of independence (consider kittens, born utterly dependent with their eyes closed, and giraffes, born able to stand and run within the first hour!), not all birds have the same needs as hatchlings. There are two main types you will likely encounter in the wild:
Altricial birds hatch with few or no feathers, and are extremely dependent on their parents for warmth and care. Most common songbirds fall into this category. These chicks must be fed an average of every 20 minutes throughout daylight hours, which must be an exhausting task for their parents. Often both mother and father take turns warming and feeding the hatchlings.
Altricial hatchlings are almost entirely bald, or with skin showing through patchy fuzz. Sometimes their eyes are still closed. If you find a hatchling and can locate the nest, carefully try to return the baby to the nest. It is a myth that the parents will smell your human scent on the baby and reject it, as birds do not have a very strong sense of smell. What they DO have is an incredibly strong parenting instinct, so if you return the baby to the nest and then stay out of sight, the parents will most likely return to resume care for it.
The majority of endangered baby birds found by gardeners fall into the atricial category, the songbirds that nest in our trees and shrubs. Often a storm or strong wind has dislodged them from their nest, or caused the entire nest to fall from the tree. Other times, a baby bird has been pushed from the nest by a sibling looking to reduce competition, or has fallen from the nest while exploring its tiny world. Careful examination may help you determine how young the bird is.
Slightly older babies who are beginning to form their first feathers are called nestlings. These babies still need to remain in the nest, as their feathers are not developed enough and their muscles haven't gotten strong enough to allow them to fly. They will not survive long if left on the ground.
Try to return them to the nest, if at all possible. If the nest isn't accessible, or is destroyed by bad weather, you can provide a temporary nest to get the baby up out of the reach of predators. Use a small, rounded basket, box, or plastic container with holes drilled in the bottom to provide drainage in case of rain, and line it with unscented tissues or paper towels. Wire or nail it to the tree, as close as possible to the location of the original nest. Place it in a sheltered location out of direct sunlight, and then stay out of sight to see if the parents return to care for the baby. They are very attuned to the calls of their young, and will return to care for them if at all possible.
The next stage of development is called fledglings. At this point, true feathers have formed, and the baby is ready to leave the nest. This is a dangerous time for the baby, as it often ends up on the ground within easy reach of predators, and is often the stage that draws concerned gardeners and homeowners. For several days, the baby can hop around and flap clumsily, but cannot yet fly to get to higher ground. This is a stressful time for the parents, as they stay nearby and feed the baby almost constantly, teaching them to locate food and survive on their own. This steep learning curve is crucial to their long-term survival, and can be interrupted by well-meaning people who try to take in the birds and care for them in their seemingly helpless state.
Perhaps the best thing you can do for them at this stage is to keep pets and children away from them. Keep cats and dogs inside, as their instincts will likely overcome any training you've given them, and it is nearly impossible to stop even well-trained dogs from attacking or picking up a baby bird. The hunting instinct is even stronger in cats, and I can attest to how persistently my cat will attempt to escape the house in late Spring when the fledglings are out.
So what can you do if you see a baby bird you believe is in peril? If it is a hatchling or nestling, you can gently pick it up and try to return it to the nest, as detailed above. You may want to wear gloves, or gently drape a towel over the baby and pick it up with the towel. Regardless, wash your hands thoroughly after handling any wild bird.
If it is a fledgling, assuming no immediate animal threat, just observe from a distance for 20-30 minutes. Stay out of sight and watch. The parents are likely around, waiting for any threats to leave before they land to avoid drawing attention to the location of their baby. Leaving the care of a baby bird to its parents is ALWAYS the best bet, as they are uniquely qualified to feed and train them. If you are concerned about cats or small children in the area, you can try to place it in a bush or small tree, out of reach of ground-level predators. If you have watched from a distance, and have determined that the parents really are NOT caring for the baby, then you have some difficult decisions to make.
Birds are notoriously difficult to care for, and there are laws in the United States against keeping wild birds in your home unless you have a specific license to care for wild animals. In addition, most homeowners are not able or prepared to feed them every 20-60 minutes all day. Most efforts to care for hatchlings and fledglings at home end up with the death of the bird.
If you determine it is absolutely necessary to rescue a baby bird, first prepare a small box, large enough for the bird to stand and move a bit. Line it with paper towels or tissues, and be sure there are several air holes. Try to minimize the excitement and noise as you collect the baby. If it seems calm and is cool to the touch, you can warm it in your hands. If human contact seems to agitate or frighten it, you can place a warm (not hot!) water bottle in the box to keep the bird's temperature up, or place a heating pad under a towel below the box on a very low setting. Call a wildlife rehabilitator as soon as possible. Most animal shelters are limited to caring for domestic animals, though they may be able to refer you to someone who specializes in wildlife care. If you are unable to locate any information in your phone book or through an internet search, you can contact the Fish and Wildlife office, the Humane Society, or the local police for information on nearby wildlife rehabilitators. Unfortunately, you may find it difficult to find a wildlife rehabilitator that will accept a common variety of bird.
Do not attempt to feed the baby or give it water by a dropper. Many well-intentioned people have drowned baby birds by attempting to drip water or liquified foods into their beaks. Baby birds get almost all of the water they need in the insects and worms they eat.
If you are committed to caring for a baby bird in your home until you can transfer them to a licensed rehabilitator, veterinarian and DG member Geoff Stein (Palmbob) recommends wet dog food for most insect-eating species. Baby birds, especially at the hatchling and nestling stages, are often difficult to identify, as they don't yet have the distinctive markings and colorations of the adults. In addition, the dietary needs of the babies can vary dramatically from the foods eaten by more mature birds. Again, a wildlife rehabilitator should be contacted to give the baby bird the best possible chance at survival!
Audobon Society of Portland, Oregon: http://audubonportland.org/wcc/urban/babybirds
The Aviary at Owls.com http://aviary.owls.com/baby_bird.html
Messinger Woods Wildlife Care and Education Center: http://www.messingerwoods.org/babybirds.htm