Summarize:

In almost all circumstances, it is in a wild bird's best interest to be raised by its parents. Being raised by humans has numerous disadvantages for the bird, especially if you intend on releasing it back into the wild, so be sure not to intervene unless necessary. A bird that is raised by humans will miss out on important social interactions with other birds, and may not be able to pick up everything it missed. A bird that is raised by humans will not learn many of the survival skills its parents would have taught it, such as how to find food or how to spot a predator. This will decrease the bird's chances of being able to survive in the wild. A bird raised by humans is likely to exhibit no fear of humans, which could get it into trouble with people who are not familiar with the bird. Whenever you find a baby bird, you should determine whether it is injured. If so, contact a veterinarian or wildlife rehabilitator right away. Signs of injury include bleeding, wings that are drooping or twerked up, an inability to flutter its wings, shivering, and falling over. It is fine to handle the baby bird. It is not true that its parents will reject it if it has been handled by humans. Be sure to always wash your hands before and after handling a bird. If the bird was in another animal's mouth (like your cat or dog), you should assume that it is injured and get medical help for it. It may need antibiotics to protect it against dangerous bacteria acquired from the other animal, even if its skin isn't broken. Do not assume that a bird is injured just because it can't fly or seems slightly clumsy. Birds typically leave their nests before the are completely able to fly. This is a normal part of the learning experience for them. If you have determined that the bird is not injured, the next step is to determine whether it is a fledgling, which means it is learning to fly, or a nestling, which means it should still be in its nest. Fledglings may not be able to fly, but they should be able to grip firmly on your finger or another perch. Nestlings will not be able to do this. You can also identify a nestling by its lack of feathers. If you find a less than fully-feathered bird on the ground, it is probably too young to be there. If you have found a nestling that has fallen from its nest, the best possible thing you can do for the bird is to return it to its nest as soon as possible. Look in nearby trees and bushes closely, keeping in mind that the nest may be somewhat hidden. If you can't find the nest, you can create a new nest for the bird by placing it in a small basket or container and hanging it from or nailing it to a tree. Make sure the container has holes in the bottom for drainage and is lined with paper or cloth to prevent the bird from slipping. If you have determined that the bird you have found is a fledgling, you do not need to return it to its nest, but you should remove it from any danger that it might be in. This includes moving it out of a road or placing it in a tree if there are predators nearby. If the bird is not in immediate danger, there's nothing for you to do. Simply leave the bird alone and let it learn to fly. If you know that you have fledglings in your yard who are just learning to fly, try to keep your pets out of their way for a few days, and ask your neighbors to do the same. If you have left a fledgling be or returned a nestling to a nest, you may still be concerned about it. If so, it's best to watch it from a distance. Its parents will not return to take care of it if you are there, so give the bird some space and wait to see if they come back for the baby. Parents should return to their babies within two hours, but they may not stay long. If you have been watching the bird and the parents do not return, you may consider intervening.
Understand what's best for the bird. Check for injuries. Determine if it is a nestling or fledgling. Return nestlings to the nest. Remove fledglings from immediate danger. Keep an eye on it.