DOI: 10.5061/dryad.ncjsxkt67
title: Data from: Discordancy of two common methods of measuring feather hydrophobicity

description: 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.
description: # Data from: Discordancy of two common methods of measuring feather
 hydrophobicity Dataset DOI:
 [10.5061/dryad.ncjsxkt67](10.5061/dryad.ncjsxkt67) ## Description of the
 data and file structure **Experimental context & data description:**
 The attached raw data explains the values found when measuring contact
 angles of water droplets on contour feathers of Swainson's Thrushes
 and Bicknell's Thrushes, both before and after washing the feathers
 to remove any dirt and/or preen oil. This data was collected to answer the
 experimental hypotheses, which was that Bicknell's Thrushes feathers
 were more hydrophobic than Swainson's Thrushes. We included an
 analysis with and without preen oil to understand if this substance had
 hydrophobic qualities. All analyses were done in RStudio, and readers may
 request the code by emailing
 [sdeckel@greathollow.org](mailto:sdeckel@greathollow.org). The data is
 organized in a .xlsx file, with columns for each variable explained below.
 #### File: Raw\_Data.xlsx | Field | Definition | |
 ---------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------- | | Sample ID | Unique identification value given to individuals in the field | | Species | BITH = Bicknell's Thrush; SWTH = Swainson's Thrush | | Hydro\_penn\_bw | Contact angle measurement taken on the pennaceous vane of the feather, before washing uropygial oil | | Hydro\_penn\_aw | Contact angle measurement taken on the pennaceous vane of the feather, after washing uropygial oil | | Index\_p | Structural index value | | \*Contact angle measurements are based on 160 degrees, where larger numbers closest to 160 indicate greater hydrophobicity.  | | NOTE: any missing values in the dataset are indicated with "NA".  ## Code/software **Code/software:** all analyses were run in RStudio (version 4.4.1), but the data can be viewed using Microsoft Excel. The only package we used was "dplyr" (version 1.1.4). If the reader would like to see the code, they can contact [sdeckel@greathollow.org](mailto:sdeckel@greathollow.org). ## Access information Other publicly accessible locations of the data: * NA Data was derived from the following sources: * NA