Tanned, dried, scraped bird skin. It is quite delicate. Without its feathers, it provides little protection from heat or cold.
Birdskin is one of the least protective and least insulative textiles in the game. It also has a fairly low beauty factor. Thus it should only be used when there is no alternative or where its stats have little effect on the utility of the resulting item, such as in clothing production quests or armchairs whose beauty rarely significantly affects a room's quality.