Cul De Canard better known as CDC is used primarily for dry flies such as the F Fly and the Moustique. The actual translation of Cul de Canard is duck butt even though these feathers are from the back of a duck directly around the preen gland and not from the butt (that doesn't stop us from calling them duck butt feathers). The preen gland secrets an oil that the duck uses for cleaning and waterproofing. This makes these feathers very buoyant.
It is very important to note that using gink or other silicone-based floatant will cause this feather to sink. If extra flotation is required, use dry dust powder or something similar. After a CDC fly is fished for a while, it may lose its buoyancy. Cup the fly in your hands and blow out the water or use an amadou pad to dry the fly. This will help the fly to float again.
A CDC and Elk dry fly, ready for the Mother's Day caddis hatch.
The use of CDC's in fly fishing originated from the Jura Mountains in Switzerland during the 1920s where fly fishermen used this feather in dry fly pattern called the Moustique to add buoyancy. In the 1980s this material gained popularity primarily in Europe.
A package of Petitjean CDC feathers.
CDC enthusiast Mark Petitjean has done lots of research in the use of CDC and is one of the primary sellers of the feathers. His work along with many others has lead to a resurgence in tying with CDC.
If you haven't used a CDC pattern give one a try. One of our favorites is the CDC and Elk Pattern.