We have 10 (now 9): girls Each one is different breed. All have been vaccinated. We've lost 3 different Buff Orpingtons. One had Meraks and the last two we are not sure but the one we lost last night was breathing heavy wtih mouth open so maybe a respiratory problem. We gave her tylan but it didn't help. We're going to take her to UC Davis to diagnose cause of death. She was 19 weeks. So to my question...All my other girls have been healthy and strong. Do Buff Orpingtons have a less strong immune system. Are they most susceptible to disease. I love Mabel so much she was so sweet. When we started getting chickens the BO was the breed I wanted most.