The feather's multi-functional structure across nano to macro scales inspires hierarchical design.

Bird feathers are finely tuned structures with key features at every length scale, from nanometre to metre, furnishing a unique multi-functional hierarchical design that can inspire material scientists, biologists and designers alike. Feathers are not only a crucial component in equipping birds with flight, but are also responsible for thermoregulation, coloration and crypsis, water repellency, silencing and sound production, sensing, directional fastening and even self-healing. Despite this broad multifunctionality, all feathers are formed from the same basic template using a universal building block: the feather keratin protein. Consequently, feather diversity across approximately 10 000 bird species arises from subtle differences in architecture rather than variations in chemical composition. To understand these underlying hierarchical mechanisms, we systematically review feather properties across all length scales, connecting development and morphogenesis to biomechanics and integrated structure-property-function relationships. This systematic distillation of the feather's complex design into comprehensive principles will enkindle new biohybrid, biomimetic and bioinspired material solutions.