DOI: 10.5061/dryad.b8gtht7c9
title: Data from: Malar stripe size and prominence in peregrine falcons vary positively with solar radiation: Support for the solar glare hypothesis

description: Many falcons (Falco spp.) exhibit a distinct dark plumage patch
 below the eye, termed the malar stripe. This stripe is hypothesised to
 reduce the amount of solar glare reflected into the eyes while foraging,
 thereby increasing hunting efficiency in bright conditions. Here, we use a
 novel, global-scale correlative approach to test this “solar glare
 hypothesis” in peregrine falcons (Falco peregrinus), the most widespread
 falcon species, using web-sourced photographs from across the species’
 global range. We found that the size and prominence of the malar stripe
 were positively associated with average annual solar radiation, but not
 with other environmental variables, such as temperature and rainfall. Our
 results provide the first published evidence for the hypothesis that this
 plumage feature functions to reduce the amount of solar glare reflected
 into the falcon’s eyes, thereby improving the ability to pinpoint and
 target agile prey in bright conditions.
description: Photograph metadata for user-submitted photographs of peregrine
 falcons (including the name of the photographer, date and location at
 which the photograph was taken, GPS coordinates, photographer comments,
 and sex and subspecies identification) were downloaded from the Macaulay
 Library (https://www.macaulaylibrary.org/) and iNaturalist
 (https://www.inaturalist.org/) online databases, using the websites'
 export tools. All photographs were then downloaded using the list of image
 URLs and filtered to include only adult birds correctly identified as
 peregrine falcons. For each country represented in the photograph metadata
 (and for each state or province, in the case of the USA and Canada), a
 random selection of 50 photographs was then chosen for analysis. This
 selection process was repeated until the maximum number of 50 usable
 photographs for each country or state was reached, or until there were no
 more photographs available. Data on average annual solar radiation
 (W/m<sup>2</sup>), average annual rainfall (mm) and average
 minimum daily temperature (ºC) at each photograph's GPS location were
 then extracted from the TerraClimate dataset
 (http://www.climatologylab.org/terraclimate.html) using Google Earth
 Engine. For photographs which did not have user-submitted GPS coordinates
 available, the GPS coordinates for the general area were sourced from
 Google Maps. All values represent 30-year (1988-2018) averages taken
 within a 20km buffer of the photograph's GPS location. Photograph
 metadata and climate data were collected between March and October
 2019. For each individual bird in each photograph, a
 single observer then scored the characteristics of the malar stripe
 according to a ten-point visual scale constructed for this purpose.
 Five aspects of the malar stripe were quantified: width, contiguity with
 the hood (extent of dark plumage between the malar area and the hood),
 prominence (darkness), length, and elongation (ratio of length score to
 width score). The same observer also scored the angle and position of the
 bird's head in each photograph according to a visual scale (see
 ReadMe for details). The dataset was then filtered to include observations
 of uniform photograph quality and angle, and from these, a representative
 sample of 213 observations was selected. Using ImageJ
 (https://imagej.nih.gov/), the same observer defined the malar stripe area
 on each photograph using the freehand selection tool, and converted these
 selections to binary image masks (Edit&gt;Selection&gt;Create
 Mask, Process&gt;Binary&gt;Make Binary). All masks were
 standardized to a width of 40px (Image&gt;Adjust&gt;Size), and the
 area and perimeter in pixels, circularity, and roundness of the
 thresholded area, as well as the major and minor axis lengths and aspect
 ratio of the ellipse fitted to each selection, were calculated using
 ImageJ's shape measurement tools (Analyze&gt;Measure). The same
 menu tool was also used to calculate the mean and median grey values of
 the initial freehand selection for each image.
description: See the ReadMe file for descriptions of the variables used in
 this dataset and package installation requirements. For detail regarding
 the malar stripe and head position scores and how these were calculated,
 see the supplementary material of the associated publication. 
 All statistical analysis for this study was performed using R
 version 4.0.3. Instructions for running the code and descriptions of
 outputs are provided in the descriptive text within the R code file, and
 in the associated publication. A list of all
 photographs analysed in this study is included as a set of appendices
 accompanying the associated publication.