I wonder if the blue dog shampoo would work. It's meant to brighten white coats. A concoction that I have always used to bleach tear stains in poodle might also be useful for chickens. You mix equal amounts of Hydrogen peroxide and Milk of Magnesia. You add this mixture to cornstarch to make a paste and you leave it on the stained area for 10 minutes. Generally in dogs this is near their eyes and feet, so you have to be very careful not to get it into their eyes and you have to prevent them from licking their feet.
If you were thinking of trying it on a chicken, I would recommend you try it on some shed feathers first to make sure it doesn't wreck the feathers.
