Data from: 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 been 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.