have you used that particular rabbit hutch before? could it be something in the hutch itself that was making the chicks sick? did you build it yourself, or get it from someone else? If it is a "hand-me-down" hutch, perhaps it had been sprayed heavily inside with a flea killer for the rabbits, and that is what was killing your chicks. My daughter raises rabbits and she sprays the inside of her hutch with flea spray. The spray residue can last for a long time and it wouldn't take much to kill baby chicks.