Well, the juglone is toxic to many plants, but I don't think it is toxic to most animals or birds. There are cases of dogs being poisoned by eating walnuts but this is said to be caused by a mold that grows on walnuts. And horses can be poisoned by walnut shavings in their bedding, causing laminitis, making horses lame and causing pain in hoof area. Walnut hulls have antiviral and antimicrobial properties, and are used in some natural health and beauty products.
But, none of these things would indicate that the leaf-eating caterpillars would be poisonous to chickens.
My personal thoughts would be, if you free range your chickens and they make choices as to what to eat and what not to eat, you could try offering one or two, and see what they do. They may just turn up their little beaks and walk away, lol!
