A couple of studies have suggested that passerine birds deliberately seek out spiders to feed their young. Obviously the bird isn't aware of it, but spiders are rich in taurine, which is an important nutrient for birds. I imagine chickens would similarly benefit from spiders in their diet. Now, I haven't gone through the effort of deliberately feeding out spiders, but I see plenty of them around the yard and I know that my hens go crazy if a fat, clumsy black widow gets into their sights. I would think that as long as the bird isn't bitten, there would be no real harm in feeding spiders. And generally with how quickly birds glom the little guys down, I suspect the bite risk is minimal.