Very interesting!
I wonder if your mareks weakens them and then a fungal infection kills them.
Copper is used to treat fungal infections
Well, that is what we are wondering also. I got on line and found an article about copper's affect on chickens and seems as though it is beneficial and completely excreted in their feces so there is not toxic build up. I then looked at the ingredients of MFA's hog feed and sure enough, it has a small amount of copper in it plus it is lower in protein. I worry that the high protein is going to go hard on their kidneys and livers not to mention make them too fat so I mix it 50-50. Watching them, they LOVE the pelleted hog food and I haven't had a sick bird in the three months since I started mixing the feed.
I know I've lost birds to systemic fungal infections so heck, why not give it a try.