Almost every single tree in my yard is a black walnut tree. haha its awful.
But I haven't ever had a problem with my chickens pecking at the hulls or anything and neither of them have ever been sick...
Black walnut hulls contain a herbicide that kills off competition from other forms of vegetation & trees. I suspect that is why so many communities are named "WALNUT GROVE" etc because there is or was nothing much but walnuts growing there.