I use lemon woodduck flank for tailing fibers on mayflies (PMDs), and gadwall on darker ones such as March Browns. Mallard flank tends to be a little soft for that purpose, and also usually pretty curved. I hear of mallard being used for trailing shucks, however. The barring is very buggy-looking.
Not every Flack Feather is usable, some have ragged tips, or large portions of the hackle is missing, There are some salvageable hackle, if you are willing to do a little extra work…
Sometimes the Flack Feathers, have to much of a curve in their hackle stem, for use in a fly pattern. But they can be flatten, to be of use.
Lay a towel down on a flat surface, and place your curved fly feather on the towel and fold part of the towel over the feather, then steam it with a steam iron. When you have steamed it enough, remove the feather, then place it between a folded sheet of wax paper, and place it under a big book.
Flank feathers have been used for years as tails on dry flies. Many of the original versions of Catskill flies had wood duck tails. Wood duck is used for tailing on many of Rene Harrop’s dry flies, in conjuction with a shuck made from dubbing. I would refer you to my recent fly of the week article, the Last Chance Cripple, but I’m not sure about the archives these days. I’ve got the fly on my own website under “techniques” as well. Flank feathers work beautifully for tailing, though I would agree floatation isn’t quite as good as hackle. However, that’s offset by the very effective mottling. Great for flat water.
Alice Conba does a gorgeous bronze mallard flank feather tail on many of her wet flies, using just three fibers. Of course, those don’t have to float.
Duck flank is plenty stiff enough for dry flies and is called for in traditional Catskill patterns. The natural barring on drake puddle ducks makes it desirable in many ways. Mallard, wigeon, woodduck, teal, gadwall and even hooded merganser have suitable feathers. With today’s superior light weight dry fly hooks and floatants there is no need for super stiff tails to float a fly.