I have 4 ducks about 8 months old. 2 Pekin female and 2 male mallards. One of the males is loosing his green head feathers. From eyes, around beak and down sides of neck. I got a poultry dust about ten days ago and applied it to just him. On the green area while his eyes and bill were covered. I applied again last night. But i haven't seen any improvement. Any thoughts???