I mix it to a slurry with water and paint it on the perches in the chicken coop. It's meant to prevent red mite infestations as it dries out the mites as they try to crawl over it. I've also dusted it into all the nooks and crannies around the coop and I toss a bit into the chooks' dust bathes so they can work it through their feathers.
So far I've had no sign of external parasites so it may be working... or I may just be lucky. No way of knowing really! I will keep using it though as it's cheap, easy and acceptible in an organic system (not that my back yard is organic).
I also used it in my chooks' food for a while as an attempt to reduce intestinal worms. They still got worms so I don't bother any more.