Data and R-code from: Migration, habitat and hunting style do not affect the malar stripe of different falcon species
Description
The solar glare hypothesis suggests that malar stripe of a falcon decreases the sun's glare in the eye, possibly increasing their hunting success. The amount of sunlight an individual experiences could be affected by its migratory strategy, hunting strategy, or main habitat. However, it is not known if these environmental variables impact the size and intensity of the malar stripe. Therefore, this study aimed to analyse differences in size and intensity of malar stripes between individuals of twelve falcon species with different migratory strategies, habitats, and hunting styles distributed worldwide. The malar stripes of twelve falcon species were measured and scored using more than 6000 photos from citizen science repositories. The measurements of the malar stripes were first reduced with a principal component analysis (PCA) and then analysed using a linear mixed model that included migratory strategy, habitat, and hunting style as fixed factors and several posture variables as random factors. The relationships between the measurements of the malar stripe and solar radiation of the individual's location were also analysed using linear mixed models. Overall, we found no differences in malar stripe size and intensity between species with differing migratory strategies, habitat and hunting styles. The relationships between various characteristics of the malar stripe and solar radiation did depend on the species and the habitat the individual occupied. Therefore, migratory strategy, habitat, and hunting style do not markedly influence the size and intensity of malar stripes across falcon species, suggesting that other mechanisms, such as thermoregulation or camouflage, also play a role.
Methods
For this study, we compared the malar stripes of 12 falcon species with differing migratory strategies (migratory, partially migratory, and sedentary), habitats (grassland, woodland, shrubland and forest), and hunting styles (aerial, insessorial and generalist). We used two online citizen science repositories, iNaturalist (iNaturalist, n.d.) and the Macaulay Library (Macaulay Library, n.d.), to get access to photos of the 12 different falcon species.
Table 1. The 12 falcon species included in this study with their migratory strategy, habitat, and hunting style.
|
Scientific name |
English name |
Migratory strategy |
Habitat* |
Hunting style |
|
Falco amurensis |
Amur Falcon |
Migratory |
Grassland |
Insessorial |
|
Falco columbarius |
Merlin |
Migratory |
Woodland |
Aerial |
|
Falco naumanni |
Lesser Kestrel |
Migratory |
Grassland |
Generalist |
|
Falco subbuteo |
Eurasian Hobby |
Migratory |
Woodland |
Aerial |
|
Falco cenchroides |
Australian Kestrel |
Partially Migratory |
Woodland |
Generalist |
|
Falco biarmicus |
Lanner Falcon |
Partially Migratory |
Shrubland |
Aerial |
|
Falco sparverius |
American Kestrel |
Partially Migratory |
Grassland |
Insessorial |
|
Falco tinnunculus |
Eurasian Kestrel |
Partially Migratory |
Shrubland |
Aerial |
|
Falco berigora |
Brown Falcon |
Sedentary |
Woodland |
Aerial |
|
Falco chicquera |
Red-necked Falcon |
Sedentary |
Woodland |
Aerial |
|
Falco novaeseelandiae |
New Zealand Falcon |
Sedentary |
Forest |
Aerial |
|
Falco rufigularis |
Bat Falcon |
Sedentary |
Forest |
Aerial |
For each species, we analysed 500 photos that were equally distributed over its distribution range. Per individual, we measured and calculated six measurements of the malar stripe: the width of the malar stripe, which is the thickness of the stripe; the contiguity with the hood, which is the connection of the malar stripe to the dark plumage on the hood; the prominence, which is the intensity of the malar stripe; the length of the malar stripe which is the maximum distance from the dorsal to the ventral end of the stripe; the elongation, which is calculated by dividing the length by the width; and the surface, which is calculated by multiplying the width by the length.
The width and length were measured using an on-screen measuring tool (Aequo), and these two measures were used to calculate the elongation and surface of the malar stripe. The contiguity and prominence were scored using a scoring template. We also scored the width and length of the malar stripe and used these scores to calculate a scored elongation and surface. Because the position of the birds varied a lot throughout the photos, the measurements were done relative to the width and height of the bird's eye. We performed each measurement three times and used the average of these measurements in the analysis. Also, the bird's posture was scored using a scoring template.
Finally, we obtained the average direct normal irradiation (kWh/m2) using a Global Solar Atlas for each region a photo was taken in. The Handbook of Energy, "Section 10 - Solar" (2013) stated that "Direct Normal Irradiation (DNI) is the amount of solar radiation received per unit area by a surface that is always held perpendicular (or normal) to the rays that come in a straight line from the direction of the sun at its current position in the sky." Hence, we used direct normal irradiation as a proxy for the amount of sunlight an individual experiences throughout the year.
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Additional details
Related works
- Is source of
- 10.5061/dryad.cc2fqz6gg (DOI)