We have several black walnut trees all over and they haven't been a problem for the chickens. I know the leaves and husks have some toxins but I don't think chickens would consume those, due to that overpowering smell they have. The nutmeats themselves, would probably be a treat if they could get at them but they don't try. I just don't want black walnut anything in the compost because it has chemical called jugalone, which has allelopathic properties, stunting other plants to eliminate the walnut's competition.
I don't know though, if there was a black walnut tree over a bare run or something where the chickens didn't have a lot of access to other things, they might start going for the green walnuts. Mine have never seemed remotely interested, but everyone's set up and birds are different.
