Reflect Light, Stay Cool | Outside My Window


Nankeen kestrel in Australia (photo from Wikimedia Commons)

How do birds stay cool in hot climates, especially when there’s no shade?

A 2018 Australian study in the journal Nature found that some birds can reflect the hottest part of sunlight, the near infrared (NIR) spectrum.

Near infrared is long-wavelength light beyond the red end of the visible spectrum. Though we can’t see this wavelength we can feel its heat. In fact more than half the sunlight that reaches Earth is in the infrared spectrum.

Most of the sunlight that reaches the Earth’s surface is in the infrared range (simplified image from Wikimedia Commons)

Australia is a good place to study cooling techniques in birds because 70% of the continent is hot, dry and very sunny. The study examined museum specimens of 90 species, classifying them by habitat and testing them for their NIR reflectant properties. Two species stood out.

The nankeen kestrel (Falco cenchroides), named for his yellow color, reflects near infrared light from the crown of his head, above. The azure kingfisher (Ceyx azurea) stays cool by reflecting NIR from his chest. Their feathers reflect NIR because they have rounder barbs and denser barbules.

Azure kingfisher, Queensland, Australia (featured picture on Wikimedia Commons)

This graph from the study compares four species. The nankeen kestrel and the azure kingfisher are at the top of the scale but neither of them reflect much visible and UV light. The blueish bird, a male superb fairywren (Malurus cyaneus), reflects UV and visible light but not NIR, probably because he lives where it’s moist and shady. The great cormorant (Phalacrocorax carbo) doesn’t reflect much light at all.

Graph of four species (image from Reflectivity article in Nature Communications)

The study also found that near infrared reflectivity is more prevalent in small birds because they benefit more for their size. The azure kingfisher is only as big as a sparrow.

Too hot? Reflect near infrared light to stay cool.

(image credits: nankeen kestrel, sunlight graph and azure kingfisher from Wikimedia Commons. Graph from “Reflection of near-infrared light confers thermal protection in birds” at Nature.com, Creative Commons license. Click on the captions to see the originals)