wild birds can carry parasites, ticks, etc. its not a good idea to touch them, however if you find a baby WILD bird fallen from the nest you can put it back in the nest if you can reach the nest. The mother will not reject it just b/c it was touched by a human. However mother birds will push out very young babies if they are ill as to not infect the rest of the babies. If she pushed it out because it was not healthy, she will do it again if you put it back and it is sick. Its not cuz you touched it, but cuz its not healthy to begin with. They also push out older babies to encourage them to learn to fly. I have put a baby robin back into a nest and it was welcomed back by the mother. She cant pick it up on her own, so I believe she appreciated the help. I have also done it for the swallows that nest in our barn every year. They are not rejected unless the mom knew it was not healthy to begin with.
Go ahead and handle the chicks. Just be gentle and kind, as others have said. They need to know humans are not going to hurt them, so they are friendlier and less aggressive as adult poultry.