I believe that the THC has to be heated to have any effects (ie, burned (for smoking) or baked (for eating)), so I don't think that there would be much, if any, effect from the chickens eating raw leaves and/or seeds. I would start with a small amount just to be sure there are no unwanted side effects, but I think they'll be fine. They may or may not even like the taste of it though, I would think it might be bitter-tasting?
