Health issue of causes one birds behavior to change. The change in behavior makes so other(s) in flock no longer recognize afflicted then treat it like an outsider. A healthy rooster can make short work of another bird that is already compromised and can not get away.
Changes can be overt behavior or simply voice, Mareks can cause both as can other conditions.
Changes can be overt behavior or simply voice, Mareks can cause both as can other conditions.