Migration, Habitat and Hunting Style Do Not Affect the Malar Stripe of Different Falcon Species

The solar glare hypothesis suggests that the 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 style 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 12 falcon species with different migratory strategies, habitats, and hunting styles distributed worldwide. The malar stripes of 12 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 analyzed using linear mixed models. Overall, we found no differences in malar stripe size and intensity between species with differing migratory strategies, habitats, 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.