My resuced bird was improvving and then died?

found a babybird-in between lawn and the crack of a sidewalk, saw no signs of a nest, brought it home fed it drops of water and a little bread mushed in water, gave this toht in extra small quantities. The poor guy was bald, but alert, his legs were exremely long, it did alot of wobbling. On the second day it began to chirp. I kept it in a card board box lined with shredded paper, during the day I kept it in the box on the upper deck of my deck. Evening i placed it in the garage, covered with a punctured paper towel. It was beginning to urinated and poop as well as chirp. I thought all of these were good signs- this morning the poor thing died and I'm heart broken what did ido wrong?