The chickens at my parents' house are "itchy & scratchy" all the time. This is new. We don't know what had happened, as no new chickens have been introduced. They are completely free-range, and they love to hang out under the wild bird-feeder. I'm thinking they might have picked up bugs from the wild birds? That, or the squirrels. So, I read about "dusting" the chickens with Sevin dust. Is this what I should do?