Help! Little baby bird in my backyard?

There are probably 2 or 3 nests in my backyard and this little baby bird fell out and he survived. My mom found a nest that no birds were using and put it in there and gave it a cap ful of water. This was yesterday. I don't think the mom will find it and I want the bird to live! What should I feed it? I know they eat food that their mom has already chewed but we don't have that! Please help!