depends whether or not it's a fledgling or one your cat got out of a nest - if it's the former the parents should be around and will take care of it for a day or two - if it's the latter, it will have few feathers and be unable to grip onto your finger very well, in which case try and find the nest and return it there. If all else fails, millions of birds have been raised with a pair of tweezers and some meelworms! If you go down this route google in something like 'raising an orphan bird'