Exploring the role of barbule density in feather water repellency: insights from 3D modeling and a comparative analysis

Bird feathers serve numerous essential functions, with water repellency being particularly critical, as many other feather functions depend on remaining dry. Despite its importance, significant gaps remain in our understanding of the mechanisms underlying feather water repellency. This property arises primarily from feather microstructure. Feather morphology varies widely among bird species. However, the functional consequences of this variation for feather wettability remain poorly understood. Advances in computer-aided 3D modeling enable us to investigate such structure-function relationships with unprecedented precision. In this study, we created 3D feather models from micro-computed tomography scans of a seaside sparrow feather and systematically altered its morphology, specifically barbule density, to observe how feathers interact with water droplets using computational fluid dynamics simulations. We quantified the spatio-temporal distribution of droplet behavior, measured spreading radii during interactions, and assessed final contact angles for feathers with varying barbule densities. We validated our simulation results with barbule-density estimates and static contact-angle measurements on contour feathers from four New World sparrow species, including the seaside sparrow, spanning a gradient in water exposure. Together, the computational and experimental results demonstrate that higher barbule density reduces water repellency. Collectively, these findings highlight how feather microstructure directly governs feather wettability.