"Feather meal results from the processing of the feathers obtained after poultry slaughtering. Feather meal is used as a source of protein for farm animals and as a fertilizer"
https://www.feedipedia.org/node/213
"Feather meal being nitrogen rich (15% N), inexpensive, and readily available sources serves as a potential substitute to guano. It not only supplies nitrogen to plants and promotes microbial activity, but also structures the soil and increase water retention capacity."
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/feather-meal
"Feather meal is a byproduct of processing poultry; it is made from poultry feathers by partially grinding them under elevated heat and pressure, and then grinding and drying. Although total nitrogen levels are fairly high (up to 12%), the bioavailability of this nitrogen may be low. Feather meal is used in formulated animal feed and in organic fertilizer.
Worldwide, approximately 50 billion chickens were used for human consumption in 2014"
https://en.wikipedia.org/wiki/Feather_meal
Feather meal, a semi-slow-release nitrogen fertilizer for organic farming
https://link.springer.com/article/10.1007/BF00748776