We have never had bumblefoot either. Five plus years and meaties and hens and roos. (~100 birds) Maybe, we live in rough country and the chicks all start playing king of the rocks as wee little sprites and by doing so, toughen up their feet so nothing could ever even think to be able to poke in their feet. Maybe? Perhaps you should add some really rough granite boulders to your run and test this hypothesis.....