I noticed that one of the silkie hens had been breathing heavily and making strange-sounding coughing noises for a few days. I was going to wait and see if it was just something that would clear up on its own, but when I went out to the coop this morning to check on her, she was dead! Does anyone have any idea what kind of sickness/disease she had? Is it contagious? How should I prevent the other chickens from getting it?