If you find a bird that is obviously injured, you should contact a local bird rehabilitator. See this list for contact information.
For more information about what to do if you find an injured bird, visit this page at the Cornell Lab of Ornithology website. Please keep in mind that it is against the law to hold a wild native bird in captivity without a permit, and anyone who is caught doing so may be charged with a felony.
The illustrator behind Bird and Moon Comics has a helpful and humorous flow chart for determining whether a baby bird needs your help.
I found a baby bird. What do I do?
At some point, nearly everyone who spends time outdoors finds a baby bird—one that is unable to fly well and seems lost or abandoned. Your first impulse may be to help the young bird but in most cases, the young bird doesn't need help. In fact, intervening often makes the situation worse.
Here's how to determine whether to take action:
The first thing to do is to figure out if the baby bird is a fledgling or a nestling.
Most of the baby birds people find are fledglings. These are young birds that have just left the nest, are still under the care of their parents, and do not need our help. Fledglings are feathered and capable of hopping or flitting, with toes that can tightly grip your finger or a twig. These youngsters are generally adorable and fluffy, with a tiny stub of a tail.
When fledglings leave their nest, they rarely return; thus, even if you see the nest, it's not a good idea to put the bird back in because it will hop right back out. Usually there is no reason to intervene at all beyond putting the bird on a nearby perch out of harm's way and keeping pets indoors. The parents may be attending to four or five young scattered in different directions but they will return to care for the one you have found. You can watch from a distance to make sure the parents are returning to care for the fledgling.
If the baby bird is sparsely feathered and not capable of hopping, walking, flitting, or gripping tightly to your finger, it's a nestling. If so, the nest is almost certainly nearby. If you can find the nest (it may be well hidden), put the bird back as quickly as possible. Don't worry—parent birds do not recognize their young by smell. They will not abandon a baby if it has been touched by humans. However, try to limit the amount of time you handle the young bird, since handling by humans can be very stressful to birds. Also, be aware that your presence at a nest may alert potential predators to its location.
If you have found both parents dead, the young bird is injured, you can't find the nest, or you are absolutely certain that the bird was orphaned, then your best course of action is to take it to a wildlife rehabilitator. A sick, injured or orphaned baby bird may need emergency care until you can get it to a wildlife rehabilitator.
Bottom line: Remember that the vast majority of "abandoned" baby birds are perfectly healthy fledglings whose parents are nearby and watching out for them.
Raptors (Birds of prey such as hawks and owls)
If you encounter a raptor that has been hit by a car or shot (or is otherwise in distress), it is best to call a rehabilitator immediately. Always use extreme caution when handling birds of prey. The talons and the beak are very sharp and strong. If you need to move the bird, cover it with a towel and gently pick the bundle up while restraining the feet. Do not put your hands in front or into the talons! They will clench automatically on any object, including your hands. Gently set the towel with the bird into a cardboard box. Be sure the bird is able to get out of the towel and close the top of the box. Do not try to hold the bird in your lap or hands. If a stunned raptor recovers, it can, and will, lash out and could seriously injure you. If available, thick leather gloves can be worn; however, do not depend on them for total hand protection.