Worldwide, five million tons of waste chicken feathers accumulate a day according to the Journal of Microbiology Research (6:2211-2222). Feathers are over 90% protein so it seems we would have found ways to use them that would make them a profitable byproduct. But not yet. Though there is a lot of research and exciting potential, nothing has really come through in a big way. So mostly the feathers are put in landfill though some is used as low-quality animal feed, costing the farmers money to dispose of feathers rather than being able to make money from them.
From my latest troll through the internet, here are some potential future large-scale future uses: shoes, semiconductors, boat hulls, concrete strengthener, high quality feed for animals and for humans, diapers, fabric and textiles, paper, moldable plastic and biodegradable plastic, insulation, beauty products, car parts, and fiber optic cables.