There are a lot of diseases that can be transmitted through poop of wild animals. Leptospirosis comes to mind. This can actually cause abortions in horses so we always try and keep the predators and wild animals away. I believe Lepto can also be transmitted to birds and humans. If you dare to do it, it may be worthwhile to do a necropsy of a few of the dead birds. Take pictures and share it with us here on the Emergency section or the meat section. There are a lot of people that know how a chicken is supposed to look like on the inside and may be able to help you find the real reason. And yes, do not introduce any new birds until you know what is going on. I believe turkeys can carry a disease that is harmless to them but causes death in chickens and pheasants. That may have something to do with it. Good luck, hope youll find the answer!