Rare Feathers
From WikiRaider
Rare Feathers, unlike normal, common feathers that can be found in nests throughout Shadow of the Tomb Raider like in the previous games, need to be collected from special birds that Lara sometimes needs to shoot down first. Of course they can also be purchased at any of the many Merchants around Kuwaq Yaku and Paititi, but they are quite pricey. Their primary use is in the Crafting of special Outfits.
Rare feathers used for crafting outfits with special abilities.
Shadow of the Tomb Raider, Survival Guide