Discordancy of two common methods of measuring feather hydrophobicity

Feather structure contributes greatly to a birds' ability to repel water, which is essential for thermoregulation and energy use. Water repellency of feathers has traditionally been inferred by measuring a structural index based on the distance between the feather radii and vane. A more direct method measures the contact angle of a water droplet resting on the pennaceous vane. This method is used for measuring the water repellency of various materials (e.g. textiles) and we considered it a standard against which the structural index can be validated. Despite widespread use of both techniques, their level of agreement with each other has not been systematically evaluated. Additionally, few studies have tested the direct contribution of uropygial oil to a feather's water repellency. We tested the correlation between the two methods, using feathers from two high-elevation species that are adapted to the cold and wet conditions of montane systems, Swainson's thrush Catharus ustulatus and Bicknell's thrush C. bicknelli. We also compared contact angles measured on feathers before and after removing their coating of uropygial oil. We found no correlation between the methods in either species, which suggests the structural index is not a reliable indicator of feather water repellency. Removing uropygial oil significantly reduced contact angles in both species, demonstrating a direct contribution of the oil to water repellency. The lack of agreement between the structural index and contact angle method may have occurred because the structural index infers water repellency by proxy, whereas the contact angle method more directly measures the degree to which a feather repels water. We consider the contact angle method to also be more standardizable than the structural index, although it requires more sophisticated equipment. We caution against continued use of the structural index and highlight the direct role of uropygial oil in enhancing feather water repellency.