They will quickly forget about it. If there are other siblings they will be busy taking care of them. If no other young they may go start another nest to raise more little ones. Many birds will raise more than one "crop" of youngsters. It's sad but that's the way Mother Nature is. Everyone has to eat.
A baby bird fell down yesterday in the warehouse I work. I put him outside and they found him but the baby went under a truck tire and a crow came down and took him. I feel bad for the parents who are still looking and calling for him. This happened yesterday. I really want to know how the birds will handle not finding the baby.