The description "Cul de Canard" was reputedly coined in the late 1950s by French tier Henry Bresson for one of his patterns. The description has contributed to some confusion, especially when it was literally translated into English as "duck's butt" or "duck's arse" feathers. In fact, the preen gland is located on the back of the bird, a short distance up from where the tail feathers sprout from the skin.

Overall (Type 1) CDC feather. Zooming in on the CDC feather structure, the stem shows, besides the barbs, jagged protrusions. The barbs in turn sport ribbonlike twisted barbules. Flattened barbules maximize the surface area. In the surface film, a larger and water-repellent surface area assists floatation of the dry CDC pattern. Zooming in even more, this illustration shows a submerged CDC feather. The ribbonlike and twisted barbules retain tiny bubbles of air that add buoyancy to the CDC. Francis Friesen Illustrations

Many birds preen, recondition, and waterproof their feathers with oil secreted from their preen (uropygial) glands. CDC feathers sit on top of the gland and the area close around it. While CDC is normally harvested from members of the duck family ("canard" is the French word for duck), other waterfowl such as geese offer feathers similar in quality. As the size of the bird increases, so does the size of the feathers.

Understanding CDC

While the natural oils in the feather assist in repelling water, the hydrophobic properties and the structure of the CDC feather are fundamental to its buoyancy. If the oil in the feather was solely responsible for making it float, dyeing CDC would prevent the feather from floating, but this is not the case--provided the dyeing process keeps the feather's structure intact.

Furthermore, CDC feathers don't float well when they are matted with water or fish slime. If the oil was the primary contributor to the feather's buoyancy, the collapse of the structure wouldn't matter, but it does.

If you can maintain the feather's structure, the surface area of the barbules in the film works to keep the fly afloat and the tiny air bubbles retained in the ribbonlike, kinked structure of the hydrophobic barbules hold up those barbs that have broken through the surface film.

A closer look at the makeup of a CDC feather shows why applying a liquid or paste floatant collapses the feather structure and ruins the characteristics that help it float.

For me, the primary quality of CDC is the mobility of the barbs, whether moving in the air currents above the water's surface or in the water currents in the film or subsurface. CDC wings positioned above the surface film do not contribute to buoyancy, but do offer a full silhouette without bulk and respond to the slightest breeze to suggest life. Submerged, the mobile CDC barbs respond to every shift in current, again suggesting life.

Natural and dyed CDC impart a built-in life to flies. This is where CDC shines and what makes it an excellent choice to feature in a broad range of patterns. CDC also blends in well with other materials, where the combinations of their respective properties complement one another for a more effective result.

With correct use of the material and treatment on the stream, CDC flies are among the most durable of patterns as well as some of the simplest to tie.

CDC Types

While CDC feathers are generally lumped together under the single umbrella called CDC, close examination shows distinct differences in their appearance, depending on where they are found in relation to the gland. Certain types of feathers are more suitable for specific purposes.

I designed and use a simple classification system to explain to other tiers the types of CDC most desirable for different patterns or functions. I categorize CDC into four distinct types:

	Type 1: This feather resembles a partridge body feather. The feather has a rounded tip and a fairly short, tapered stem with barbs set at approximately 60 degrees from the stem. I wrap this feather around the shank to produce body and trailing filaments for the CDC&Elk.
	Type 2: This feather has a thin stem with the barbs running mostly parallel to it, ending in a square, brushlike tip. These feathers are good for wings, including wingposts and loop wings in patterns such as the Snowflake Dun (Roman Moser), the CDC Micro Caddis (Ronald Leyzen), the CDC No-hackle, and the CDC Loop Wing Emerger.
	Type 3: This one is called the nipple plume and is sometimes referred to as oiler puff. This short feather lacks a discernible stem but looks similar to a Type 2 feather. This feather is ideal for tails, trailing shucks, and emerging wings for such patterns as the Snowflake Dun, the Balloon Emerger (Roman Moser), and various RS2-type flies.
	Type 4: This feather has a long stem with relatively short barbs. Shop-bought bulk packages mostly hold Type 4 feathers. I use these to tie IOBO Humpy patterns and to wrap the body on large-hook-size versions of the CDC&Elk, such as in the CDC&Elk Streamer or the Bonefish CDC&Elk (Paul Slaney). René Harrop uses Type 4 feathers for downwing flies such as his CDC Transitional Caddis.