Do you think you have an injured or orphan bird?
If the bird is just stunned, perhaps from flying into a window, put it in a dark box or a paper sack and keep it quiet for 30 minutes or so. It would help if the box or sack had sturdy pieces of wood for the bird's claws to curl around. In most cases, this is all the first aid necessary, and you may release the bird to fly away after it has had some peace and quiet in which to gather its wits.
If the bird is injured, it is important to get care for it AS SOON AS POSSIBLE! Broken bones that have dried out will not grow back together, and hollow bones dry very quickly. Call a veterinarian for advice.
There are a number of people who are licensed by both the state and
federal government as wildlife rehabilitators.
of rehabbers can be found here.
If you think you have an orphan, examine the young bird again. If it is naked, or if you can see lots of skin with feathers all still in their sheaths, and its eyes are not open, the best thing to do for it is to put it back into the nest from which it came. The second best thing is to put it into another nest with young birds of about the same age, preferably of the same species. Cardinals are especially good parents and will sometimes take care of nestlings that are not their own, but don't put great hulking starling chicks in their nests. A starling chick will kill or simply outcompete the cardinals' own nestlings.
Keep in mind that the orphaned chick's parents, or its own siblings, may have tossed it out of the nest for some reason that made sense to a bird's mind. Handling the baby bird will not cause it to be "outcast because of human scent" because birds have almost no sense of smell. However, handling the fragile baby bird could easily cause bruising and internal damage if you don't know what you are doing.
If the baby bird is fairly alert, and mostly covered with feathers that have sprouted from their sheaths, it is at the stage of development where it is called a "brancher". The best thing you can do for a brancher is to leave it alone! You can also help by keeping cats, dogs, and would-be helpful humans away from the area for a day or two.
The young brancher is a chick that has outgrown the nest, and it must now learn to fly and take care of itself. A new brancher will crouch nearly motionless, hiding from predators, and waiting for its parents to bring it food. The parents do not bring the food directly to it; they land within sight, but they make the young bird hop and flutter to get to the food. Soon the brancher will begin to pick up items in its beak, spitting them back out again if the items are not acceptable as food. The young bird will begin to tag after its parents as it grows stronger and more confident. Eventually the harassed parents will be followed closely by the screaming youngster until it learns to find its own food.
If the baby bird is not a brancher, and you cannot get it back into a nest, its chances of survival are not greatly improved by being raised by humans. Even with the best of care from their own parents, not all baby birds make it to adulthood. For advice on raising a truly orphaned baby bird, please consult a veterinarian.
Remember that the only wild birds that are not protected by law are the non-native rock doves (also known as pigeons), starlings, and house sparrows. Federal and state laws prohibit keeping native birds as pets. In fact the laws prohibit keeping even a single feather from any native bird unless you have a special permit to do so. The laws are written that way to prevent native birds being killed for their feathers.