If the eggs fell out of the nest the parents will take care of them on the ground. Don't touch them especially if they have hatched. If the parents know that the babies have been tampered with they won't care for them.

It depends: did you find them in a nest/birdhouse? If so, leave them there. The parents gotta eat, so they leave from time-to-time to get some food. They have not abandoned them. If they're on the ground, do what Vixie said. :-)