Just found a baby bird on our patio help!?

Ok we have 2 little pine trees by our pole barn and my grandma walked out on the patio and there were two dead baby birds and one alive. The cats killed one and ate the other. We found the nest and we thought the best thing to do was put it in its nest and set it in the pine tree that is a little bigger that the other. So far the mom or dad bird has been looking for its babies but i don't think it knows that the baby in the nest is its after being touched by humans and cats. The mom or dad flew by the nest but just flew away again what should we do? :(