Will a mother bird come back to take care of its 3 week old baby bird.?

There were 3 birds that fell out of their nest(I think they're called fledgings) and my cat killed 2 of them but I saved the other one. Its entirly covered in feathers but only down feahers and I put it in a cardboarbox with grass and some leaves. Its always chirping and I have to feed it every 15 mins.( worms). I've only been doing it for about 5 hours now but I just want to do what I want to do, not feed this bird. Its mom or dad or whatever have been sitting in the trees around and making noises. I don't know what to do.