Can you tell if it’s a fledgling or adult? An adult will probably pick itself up and leave on its own when it is ready, but a fledgling is less likely to. I know it sounds scary and risky, but put it in a strawberry basket (the ones that supermarket berries come in) and tie it in a tree branch and try to keep an eye on it if you can. A parent birdie may come by and collect it. If you’re intent on keeping it so it doesn’t get hurt, put a dry towel in a warm, dark place and let it nest there. Try to feed it seeds, tiny pieces of fresh fruit, and bugs (meal worms, crickets…available at pet stores) with a tweezer and water with an eye-dropper. If it won’t eat, though, there’s not much you can do for it besides wishing it luck and trying to get it to fly away outdoors.
I took in a young fledgling Thrush/Wren (I can’t tell what she is!) about five weeks ago and she’s doing much better that I thought she would. Baby animals die of shock very easily, so I was thrilled when she took food and started flying. I’m pretty sure she was kicked out of the nest because she has poor eyesight, so she’s staying with me for now on. I don’t usually think it’s ethical to keep a wild animal, but she wasn’t really meant to live and I have taken her out of the gene pool… She wouldn’t be able to earn birdie-type behaviors, like bug-caching and nest-building and such, without her parents, right?
Best of luck with your Sparrow!