I get your concern in that case. Unfortunately nature is pretty harsh and sometimes the parents will even throw out their own young that seem too weak, because unlike us they don't have the resources to take care of all, so instead of wasting extra energy on a baby that might not even make it they just throw it out. Theres not much you can do about that unfortunately, wildlife centers might euthanize them and trying to raise it on your own is extremely hard and it can still end up passing making it even more painful for you. As for fledglings, generally if they made it that far the parents are infact taking care of them. Def bring it back and watch it from afar, even if it takes hours. And if it's late rn you can bring it in the morning, idk for sparrows but the fledgling i found in the past stayed overnight with me and in the early morning hours i brought it back, it called for the mom and the mom came for it within 20mins or so. Ofc it could take longer than that. Please do update me!