Did exactly the same about 15 years ago...rescued a little baby bird from the jaws of a cat, bought him inside and looked after him.
Luckily, at the time, i had a huge mixed aviery with cockatiels, canaries and finches so i had plenty of food and cages.
Not knowing what the bird was, i had fed it morning noon and night with mashed up bread, a little olive oil and some high protein egg food for canary babies.
Even when it turned into a nice plump starling, i felt a tremendous sense of satisfaction when i let it go.