If you want something that's labeled ONLY for chickens you'll have to pay $40 for the Frontline Spray (which has the same identical ingredient as the Frontline drops). Very little is specifically labeled for chickens because pet chickens aren't big business. Almost everything used for chickens...
On the skin on the back of their neck. Sorry, I should've mentioned that.
Those are the 2 things I specifically tested myself on birds I got from auctions and swaps to see if they work. The morning of the 3rd day all bugs were gone.