Photos Reveal Purpose of Black Facial Markings in Peregrine Falcons

By Kathi Borgmann
Peregrine Falcon

Peregrine Falcon Falco peregrinus

  • Vermont, Chittenden, United States

Many animals have black markings under the eye that are said to reduce the amount of glare entering the animal’s eye, improving their ability to see especially in bright conditions. Athletes also subscribe to this idea, often placing black grease or strips under their eyes to reduce glare from the sun or stadium lights. Until recently this idea, termed the solar radiation hypothesis, went untested.

Researchers from the FitzPatrick Institute of African Ornithology and the University of Witwatersrand, South Africa sought to better understand why falcons have black markings under their eyes. They turned to photos in the Macaulay Library and in iNaturalist to test this long-standing hypothesis about plumage markings for the first time on a global scale. Peregrine Falcons made the perfect subject due to their global distribution and variability in thickness of black markings under the eye across their range.

If the solar glare hypothesis is true, the researchers suggest that Peregrine Falcons that occur in areas with higher than average solar radiation should have larger and darker marks below the eye. Alternatively darker eye markings could result from other climate related variables. Gloger’s rule posits that darker plumaged animals occur in wetter areas whereas Bogert’s rule suggests that animals in colder climates should have darker plumage.

To test these hypotheses, researchers examined over 2000 photos of Peregrine Falcons, 1843 of which came from 1200 eBirders who archived their photos in the Macaulay Library. They visually scored the length, width, degree of connection with the hood, and darkness of the black markings under the eye known as malar stripes. For each photo they also collected climate variables based on the reported location of the photo.

Peregrine Falcon
A Peregrine Falcon with a wide malar stripe in Australia where there is a large amount of solar radiation.

Peregrine Falcon Falco peregrinus

  • Western Australia, Merredin, Australia

They found that malar stripes were wider, darker black, and more contiguous with the hood for Peregrine Falcons that occured in areas that experience higher than average amounts of solar radiation supporting the solar radiation hypothesis. They found little support for Gloger’s or Bogert’s rule, suggesting that malar stripes on Peregrine Falcons may have evolved to reduce the amount of solar radiation entering their eyes. However, the researchers caution that their study relied on subjective measurements and did not account for migration which could impact the correlation between malar characteristics and amount of solar radiation experienced.

Even as such, the question addressed by Michelle Vrettos and colleagues highlights the unique ways in which photos in digital archives can be used to test hypotheses at a global scale quickly and efficiently. Keep those photos coming!

Peregrine Falcon
A narrow malar stripe on a Peregrine Falcon photographed in Canada with less solar radiation.

Peregrine Falcon Falco peregrinus

  • Alberta, St. Paul, Canada

Vrettos, M. C. Reynolds, and A. Amar. (2021). Malar Stripe size and prominence in peregrine falcons vary positively with solar radiation: support for the solar glare hypothesis. Biology Letters 17:20210116.