Baby birds fall out or blow out of nests. If you can find the nest you can return the baby.
Remember the baby didn’t fall far from the nest. Some species (consult a wildlife rehabilitator that takes in birds) will take care of two nests if you make a nest and place it nearby the other nest. This may be necessary if the original nest is too high. You can make a nest from a plastic basket from cherry tomatoes. Fill the nest with dry bedding such as leaves or dryer lint. It is important to use enough bedding to keep the baby snug. Their legs are soft and need to form properly which they can’t do if the babies aren’t packed tightly together.
A bird with improperly formed legs can’t hop to eat and can’t hop to fly and will probably have to be euthanized. Place the nest where it has protection from sun. When a bird leaves the nest it flutters to the ground. If you put it back in the tree it will flutter to the ground again. The parents will feed them and protect them. This is the time to keep your cat in the house. Birds also make nests in propane fill caps, which workers removed nest and all. Stuff a rag into the cap to prevent this. Reuniting bird of prey babies is species specific and needs to be handled by an expert.