Spain has a very rich history of fly fishing - especially in the northern mountain region. Anglers from all over the world have coveted these feathers for generations because they're the best you can get. Some believe that it has something to do with the soil & environment in Leon that made these chickens grow special feathers. At any rate, these extremely desirable feathers were rarely available to fly tiers on the market until Whiting Farms in Colorado got involved in breeding these birds.
Below is a video taken on Whiting Farms explaining all about how they got into raising these birds & all about their special feathers. To anyone who ties their own flies, this is very interesting.