Neem oil is also supposed to be effective and a solution can safely be sprayed on your chickens as well as all through your coop (I used 1 teaspoon per litre/quart warm water). It will kill insects (but doesn't hurt bees) and I've been adding it to my bucket when I mop my floors to get rid of fleas. I know it worked as the first time they must've all jumped on me for a last meal! I've also been using it on my vege garden to keep away Passion Fruit Vine Hoppers that sucked my plants dry last year. There's no waiting period before you can eat your produce and it doesn't hurt birds, fish or other animals. So it's pretty natural and safe. It just doesn't smell overly pleasant but the smell quickly disappears. It might be worth spraying it round your coop every now and again to keep creepy crawlies at bay.