It sounds like a fledgling bird - not a baby, but not quite an adult yet.
I'm sorry to say this hun, but few birds whom have been attacked by cats rarely make it.
The best thing to do is to make sure it's safe and warm comfortable and hope for the best.
Look up images of birds and see if you can find a match, see what they eat and try to accomodate to it's needs.
Just remember though hun, there is only so much a person can do and it the little guy doesn't make it, it's just nature.
Please don't be hard on your cat either hun.
I know it's easy to be angry with it for attacking a young creature, but it's in it's nature. It's what cat's do.
I hope things turn out okay!