Differences in Microstructure Morphology Results in Variable Wettability Across Feather Types in a Terrestrial Bird Species

Abstract

Feathers might be best known for the pivotal role they play in powered flight, yet they also serve to create a bird's protective barrier to the external environment. This, in part, includes repelling water and keeping birds dry. We argue feather water repellency is among the most crucial feather functions as many other functions rely on dryness for success. All birds interact with water to some degree, and they all evolved from a terrestrial ancestor, suggesting that the feathers of even the most terrestrial birds should have the basic structures required to keep water from penetrating to a bird's skin. Most feather water repellency studies have focused only on aquatic groups, ignoring its necessity in terrestrial birds. Additionally, most use only one feather type, typically the breast feather, assuming that wettability is the same over the whole surface of the body despite feathers differing structurally rather extensively across the body of a bird. Here, we directly measure feather wettability and multiple aspects of microstructure morphology of different feather types across the body. We focus on one species, the Cooper's hawk (Accipiter cooperii), a medium-sized, terrestrial raptor that has minimal exposure to water. We find that even terrestrial birds have hydrophobic feathers, yet wettability varies across different feather types. We also found correlations between barbule morphology and wettability, suggesting barbules play an important role in how feathers repel water. This study provides a baseline understanding of feather morphological variation across a bird at the most basic need for water repellency.