You have cats you found a baby bird that fell out of his nest what to do?

You can try to replace the nest where the cat removed it from, but unfortunately the sparrows are probably going to die - I don't think the mother sparrow will accept them now that the nest smells like human and cat. This is a really good lesson in the cruelty of Mother Nature - the cute baby birds are still prey for cats and other predators, and not all of them survive regardless of what humans think is fair or just.