What do we really know about the water repellency of feathers?

Feathers are complex integument structures that provide birds with many functions. They are vital to a bird's survival, fundamental to their visual displays, and responsible for the evolutionary radiation of the avian class. Feathers provide a protective barrier for the body; their water repellency is a key feature. Despite hundreds of years of ornithological research, the available literature on how feathers repel water is both limited and puzzling. Most hypotheses from the early 1900s suggested uropygial gland oil provided feathers with a hydrophobic coating. Subsequent studies showed that the feather's hierarchical structure creates a porous substrate that readily repels water with or without oil. Numerous studies and methods have been published attempting to explain, quantify, and compare the water repellency of feathers. Many overlook the role of barbules and the effect of their variation, which both likely play a crucial part in water repellency. The goal of this paper is to synthesize this research to better understand what has been done, what makes sense, and more importantly, what is missing. Previous reviews on this subject are mostly over 30 years old and did not use modern methods for systematic review. Here, we performed a systematic review to capture all relevant published papers on feather water repellency. We emphasize the crucial role of barbules in feather water repellency and why their morphological variation should not be ignored. We answer the question, what do we really know about the water repellency of feathers?