sorry you are having problems with your Cornish. Put the chicks in the tractor during the day when the sun is out and back in the garage at night. Nothing wrong with what you are feeding, they just do not have the room to burn off all of that protein. When they weather improves, leave them out. They need sun, air, and more room to move. Cornish are weather sensitive so I understand why you are leaving them in the garage. It sounds like you are doing nothing wrong..and the weather is preventing you from moving them into a better area permanently.