I have several 15 week old chicks sneezing and some have developed a cough/rattling sound. Its hard to describe the sound. They are all eating and drinking well and dont seem to be lethargic. However the sneezing has progressed to this new symptom. Their is no discernable discharge from the nostrils and there are no other apparant symptoms. I feed medicated chick starter to them. Any thoughts on what it might be and how I should treat? The affected birds are black copper marans and blue orpingtons.