Yes chickens can carry blackhead with no symptoms. I feed my turkeys apples all of the time, though I would hesitate to feed them onions. If blackhead is a problem where you are, it can also be carried by earthworms after they eat the chicken poop. I believe I heard that a good worming schedule helps with it? I have never had problems with it, so really am not an authority on the subject. Does anybody else in your area raise turkeys? Did you get your chickens as chicks or as adults?