Ok, we had mycoplasma a while back. We gave the whole flock duramycin for 10 days and all but one survived. One of our battery hens died. Now, every two or three days we go out to the hen house and find a dead chicken. It is always a white leghorn battery hen. Nobody appears to be sick. All of my other ones are absolutely fine. I have about 30 8 week old chicks and 3 black austrolops, 2 RIRs 3 BR and assorted others. Why are the leghorns dying?