If the baby bird is not injured, has not suffered from exposure, and you know where the nest is (and can reach it), place the baby back in the nest. It is a fallacy that touching a wild baby will cause the mother to reject or kill it.

This baby bird is injured, or dehydrated, cold and lethargic, or has been exposed to the sun and is too hot, it needs to see a rehabilitator immediately

I generally know where the baby came from, but cannot find or reach the actual nest,