Cul De Canard (French for "Duck bottom") are the feathers from the back of the of a duck

Duck

Duck is the common name for a large number of species in the Anatidae family of birds, which also includes swans and geese. The ducks are divided among several subfamilies in the Anatidae family; they do not represent a monophyletic group but a form taxon, since swans and geese are not considered...

 directly around the preen gland; they are very buoyant due too preen oil prduced by the preen gland. They are used when tying dry flies

Artificial fly

An artificial fly or fly lure is a type of fishing lure, usually used in the sport of fly fishing . In general, artificial flies are the bait which fly fishers present to their target species of fish while fly fishing...

 for fly fishing. They owe their buoyancy to their proximity to the ducks preen gland which secretes an oil distributed by the duck as a cleaning and waterproofing measure. It is unnecessary for the angler to add additional oil by way of floatant. Flies incorporating CDC are easy to dress requiring no floatant and popular with angler

Angling

Angling is a method of fishing by means of an "angle" . The hook is usually attached to a fishing line and the line is often attached to a fishing rod. Fishing rods are usually fitted with a fishing reel that functions as a mechanism for storing, retrieving and paying out the line. The hook itself...

s and trout

Trout

Trout is the name for a number of species of freshwater and saltwater fish belonging to the Salmoninae subfamily of the family Salmonidae. Salmon belong to the same family as trout. Most salmon species spend almost all their lives in salt water...

alike.