yeah, like Viper said try giving it some water or maybe some food like bread crumbs or something.
Where do you have it, in a cage or something?
Presumably it's a small bird - sparrow-sized? Not pigeon-sized. I've never had a cat so I don't know their habits, but it seems that trying to catch anything bigger than that would be problematic for most cats.
I don't know, this is a tough situation, because on the one hand you hate to see anything suffer, and on the other, it's just a stupid little bird. Related story: We have a mouse problem in our apartment, and my roommates put down some of those sticky mousetraps that the mice run onto and just get stuck there. It's really a pretty cruel way to kill them, since they just sit there, stuck, unable to move, until they starve or something. (Unlike normal mousetraps that snap down and presumably crack their necks.) Anyway, one day we found one that had two mice stuck to it (you know the second mouse was like DOH!) and they were both alive. I felt sorry for them and wanted to set them free (OUTSIDE where they belong of course), so (after putting on a glove to avoid diseases) I tried to un-stick them. Didn't even come close to working, the stuff had like bonded to their skin. I could have pulled them off, but only by leaving thier skin stuck to the mousetrap. So since I was obviously hurting them and they were obviously scared out of their minds, I just pulled their heads off to end their suffering right there. Tough decision but I still think it was the right one.
My roommates thought that I was insane when I tried to set the mice free, and they thought that I was really insane when I tore their heads off! (It's really easy to do with such a small animal.) They also thought that it was contradictory, but I think it was not. In both cases, I was trying to ease the suffering of the mice.
Not that I'm saying that you should kill this bird, but sometimes you have to wonder whether the bird's quality of life during and after rehabilitation will be worth it. Especially if it's a common little bird. I mean -- does it have a family? Perhaps a nest, little babies to take care of? Will they starve if it doesn't come back? Will they starve if it stays in the bird place for a week? What if it takes a week to heal, and it comes back and all of its babies are dead? Will it be sad? (Can birds be sad? I have no idea.)
Of course this is all silly thoughts and speculation, but... I don't know. It's so hard to be pulled two separate directions, one toward compassion and the other toward letting nature take its course.
Okay, enough AT for today. I just start blathering on and I can't stop myself. Must... resist...