CUL DE CANARD FLY TYING FEATHERS grow around the preen gland of ducks and geese ,which secretes the oil ducks and geese use to preen and waterproof their feathers. The oil and the water-resistant properties of CDC feathers make them great for emergers, nymphs and dry flies that ride naturally in the surface film. Unfortunately, once CDC is dyed the oil loses all of its desirable qualities. That's why our premium CDC is re-treated with preen oil after dyeing! The Fly Fishing Place offers two types of CDC: Premium CDC Oiler Puffs and economical CDC feathers.