Ryan S Terrill, Allison J. Shultz

Abstract
The ability feathers have to perform many functions simultaneously and at different times is integral to the evolutionary history of all birds. Many studies focus on single functions of feathers; but any given feather performs many functions over its lifetime. Here, we review the known functions of feathers and discuss the interactions of these functions with avian evolution. Recent years have seen an increase in research on the evolution and development of feather functions because of an increase in high quality fossils with preserved feathers, new tools for understanding genetic mechanisms of feather development, new tools for measuring and analyzing feather color, availability of phylogenies and phylogenetic comparative methods, and an increase in interest in feather molt. Here, we aim to review how feather functions interact with avian evolution, with a focus on recent technological and discovery-based advances. By synthesizing research into feather functions over hierarchical scales, we aim to provide a broad context for how the adaptability and multifunctionality of feathers have allowed birds to diversify into the astounding array of environments and life-history strategies. Overall, we suggest research into avian evolution that involves feather function in any way should consider all aspects of a feathers’ functionality, including multiple functions, molt patterns, ecological/mechanical interactions, and feather wear over time. With this more holistic view, processes such as the evolution of avian coloration and flight can be understood in a broader and more nuanced context.