Um, go to your local pet store and tell them your situation. Ask them what you can feed the bird, and do some research on doves! But keep it warm!
If your cat found it on the ground, then it either is a fledgling, or it was pushed out of the nest. I know with my zebra finches, that once zebras fledge, unless they don't fly, they don't go back up to the nest. Depending its age, it might actually be a fledgling, and it might fly off anyway. But I don't know, unless you post a pic.
If you are really worried, take it into a vet, and ask for help. Knowing vets, they will only put it down, because what are they going to do with a non-native bird? Ask them how to look after the bird.
Good luck, and I hope the birdie is ok!