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

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.

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.

Here is a simple classification system to explain to other tiers the types of CDC most desirable for different patterns or functions. CDC can be categorised 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. 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, the CDC Micro Caddis, 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, 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. 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.  Type 4 feathers can also be used for downwing flies such as the CDC Transitional Caddis.

The best quality CDC comes straight off the bird. The harvesting process is simple and swift and the average mature bird provides between 70 and 100 usable feathers.

Once you lift the cover feathers, you can easily locate the preen gland by feel as well as sight. The visible part of the gland shows up like a shiny pebble protruding from the surrounding skin and is capped by a clump of feather puffs (Type 3 feathers, or oiler puffs) saturated with oil. On the illustration, these feathers are darker and are just below the thumbnail. The larger feathers surround the gland and increase in size as they get farther away from the center. On a mature mallard the stem on the longest feathers that still retain the CDC structure may be close to 2 inches long. On a goose they may exceed 3 inches.

Store the saturated oiler puffs with the rest of the feathers and in a few days the oil will disperse evenly throughout the feathers, leaving the oiler puffs fluffy.

Fresh CDC feathers are mostly free from vermin, but to be safe put the container with feathers in the freezer for at least two days to kill any mature bugs. Some eggs may remain intact, so remove the container from the freezer for a day or two to allow any surviving eggs to hatch, then put it back in the freezer for two more days to finish the process.

Tips and Tricks With CDC

Bleaching. Bleaching natural dun-colored CDC feathers in a mixture of equal amounts of 3 percent hydrogen peroxide and household ammonia results in a wonderful, warm, light-amber color. The timing is not critical and may range from several hours to an overnight soak. Rinse the feathers in fresh water and let them air dry. The resultant feather stem remains pliant and the bleaching process appears to leave the feather structure mostly intact. The hydrogen peroxide and ammonia mixture gives off unpleasant and unhealthy fumes, so do this in a well-ventilated area.

Dubbing. Barbs broken away from the stem make nice dubbing material. Use them alone or mix in other natural fur or synthetic dubbing.

Bodies. Roll a Type 2 feather on a sheet of firm foam. Press the feather down with your fingertips and roll perpendicular to the stem. Start from the butt and work up toward the tip. Once you form the "rope," you can tie it in by the tip and wrap it around the shank for a buoyant and naturally tapered body. Many of Marc Petitjean's patterns feature this style of body. Including the stem makes these bodies virtually bulletproof, without the need of a reinforcing rib.

Trimming. When you cut CDC with scissors, you get an unnatural-looking square edge. "Tear away" the excess length of the barbs for ends that resemble the natural tips.

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