Dirt????
I can assure you that there is no dirt on those feathers. They are mainly used to tie wet flies and for this reason they are so appreciated. They are unique and so the patterns on which they are used.
If you are interested in some information about CDL feathers (types, quality, patterns, step by step, etc.), you can have a look at the following link http://webs.ono.com/moscaahogada2/pluma.html . It is in Spanish but you will find a lot of very interesting pictures, and for those who can read Spanish, a lot of information.
Hope to be of any help.
Andr?s T.