What can I feed a baby bird?

I recently found a nest of baby birds and a few days after I found it, 3 of the birds were on the ground and had fallen from their nest. About 2 days after that, all 3 of the birds were dead. Yesterday I found another baby bird from that nest on the ground, and noticed that it was a fledgling, so I decided to leave it alone. Earlier today I saw the bird in the exact same place, but it was injured and I hadn’t seen any signs of the parents. I decided to put it back in the nest anyway, because I thought that maybe the parents would come back, but they never did. I also just discovered another baby bird in the nest and they are both starving, and have had their beaks up in the air all day. I know I should take them to some kind of shelter or something, but the closest one near me is an hour away and I don’t think the bird’s injury is that painful for it anymore, because about an hour ago I checked again and it was flapping it’s wings like it was trying to fly again. Both of the birds are still too young to get their own food, which means I need to feed them or they will starve to death. I fed them crushed up grapes earlier, but I don’t think that they can survive just on grapes. Is there any food I can feed them, that won’t cost any money? I have cat food, but I’m not sure how I’m supposed to feed them that. Please help!

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