If you find a nestling bird on the ground and it's not injured, if at all possible return it to its nest. A nestling cannot walk, hop or fly, so chances are its nest is very close by, maybe even directly above where you found it. It is not true that parent birds will reject their young because they've been touched by humans. Birds in general have no appreciable sense of smell, and parents will readily accept and raise their young when replaced in the nest. Altricial birds like robins, grackles and mockingbirds commonly nest in people's yards. They build sturdy nests, but often after heavy rain or wind storms, an entire nest and its contents may be found on the ground. If the nestlings are uninjured and the parent birds are still around (they usually are as birds are very reluctant to abandon their young), the fallen nest can be replaced. Use a piece of wire mesh, like hardware cloth, cup it into the shape of a nest and secure it in the same place as the original nest. If the original nest is usable, place it in the wire mesh and return the nestlings. If the original nest is not intact, a substitute can be made using a small berry box (with drainage holes) lined with portions of the original nest and dry grass. After replacing the nestlings, watch from a reasonable distance to make sure that the parents return.
Some birds, such as woodpeckers, bluebirds, chickadees and screech owls, are cavity nesters. They do not build open nests but use holes, generally in trees, as breeding sites. Their nestlings also can be returned to the nest and will be accepted, but it may be more difficult to locate, reach or properly identify the nest site.
If you find a nestling that is injured, cannot be returned to the nest or is really orphaned with no parents in attendance, then you have no choice — rescue it. Here's how.
Gently pick it up in your hands. If it's cold to the touch, warm it in your hands until you can get it into a warm environment. Warming the chick is very important. Because young nestlings lack insulating feathers, they can quickly die from exposure.
If you can't get it to a rehabilitator immediately, you can make an adequate temporary home for a nestling by using a small container (a berry box or bowl) lined with layers of facial tissues. To keep the bird warm and dry, place a heating pad, wrapped in a towel, under the bowl and place a thermometer on the towel. Try to maintain a temperature of between 85 and 90 degrees F.
An acceptable diet for altricial nestlings for short term use is high protein kibbled (dry) dog food soaked in warm water until it's soft. This can be fed to the nestling, in bite-sized chunks, using a narrow spoon handle or any small blunt object. A healthy nestling will open its mouth and beg for food when hungry. Place the food all the way in the bird's mouth and it should swallow normally. Three or four mouthfuls are usually enough. Most nestlings will stop begging when full. Be sure to keep it clean. Wipe any excess food from the bird's face or feathers with a moistened tissue. Feed it at least hourly during the day. The food should be moist (but not runny) and that should suffice for the short time you'll have it.
Do not give it water. In the wild, parent birds have no way to bring water to their young. They get all their moisture from their food. Water put in a baby bird's mouth could go down into the lungs and kill it. And don't give birds milk; they can't digest it.
Keep the nest box clean. Replace soiled tissues often. Get it to a avian rehabber as soon as possible!