How birds send heat into space measured for the first time—a hidden reflectance of feathers

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As human-caused climate change continues to raise temperatures across the globe, understanding how birds regulate their temperature is vital for their conservation. But how much heat birds emit—an invisible spectrum of radiation known as mid-infrared—has never been studied, until now.

Published in the journal Integrative Organismal Biology, a groundbreaking collaboration between material engineers and museum biologists explored the impact of mid-infrared on birds for the first time in history, reflecting the hidden prism of light, heat, and color in bird feathers.

It's long been known that habitat plays a role in bird coloration, a phenomenon described by biologists through things like Gloger's rule, which predicts that animals like birds living in hot, humid areas will be visibly darker than those in dry, cool areas. Color is part of the electromagnetic spectrum, a visible wavelength that humans can see part of (the visible spectrum), and birds can see even more of (the ultraviolet spectrum), but heat, or infrared, exists outside the bounds of what either humans or birds can see.

Infrared is broken down into the heat animals absorb (near-infrared) but not the heat they emit (mid-infrared). The interdisciplinary team of scientists measured both in the new study.

"Relatively little attention has been paid to potential thermal adaptations in the near- and mid-infrared; yet, studies like ours help show that there may be important variation beyond just what we can see, visually," says co-lead author Meghan Barrett, Ph.D., Assistant Professor of Biology at Indiana University.

"A 'hot' topic in thermal engineering is to create passively cooling structures, and it's no secret to engineers that nature contains some of the most optimized, multifunctional adaptations that we would want to replicate. In order to uncover what it is about animals that allows them to manage their thermal loads, collaborations like this one are required to share interdisciplinary knowledge with one another," says co-lead author Thomas Lee, Ph.D. Candidate at UCLA.

Researchers from UCLA's engineering department provided crucial technical expertise and advanced, specialized instruments like spectrometers that are typically beyond the reach of biologists. The study opens the door to further interdisciplinary research exploring bioinspired design along with birds' ability to cope with rising temperatures.

"It's hard to get access, and also many engineers don't want dirty biological materials in their very fancy, expensive equipment," says co-author Dr. Allison Shultz, Curator of Ornithology at the Natural History Museum of Los Angeles County.

The research team measured the mid-infrared and near-infrared reflectances, as well as the visual and ultraviolet spectrum (which birds can see) of five species of birds from three regions across North America: the great horned owl, Northern bobwhite, Stellar's jay, song sparrow, and common raven.

For each species, the team examined museum specimens from geographically diverse areas across North America, representing regional subspecies of the five birds. Out of the five, bobwhites showed the most variation in heat emittance, suggesting one big factor influencing mid-infrared radiation in birds is their exposure to the vacuum of space.

"Whenever you go outside, and you don't have a ceiling, a roof, or a tree over your head, because space is so cold compared to Earth, heat is being emitted into space," says Shultz. Bobwhites typically prefer open prairies and grasslands, making mid-infrared more impactful on their survival.

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"If you live in the forest and you're never exposed, mid-infrared might not be a really big selective pressure. But if you're living out in the open, if you're a grassland bird, for example, you are exposed to the sky quite a lot of the time. So that might be a larger selective pressure for you."

Comparing the birds' absorptance of near-infrared radiation also revealed some surprises. "When we divided up the common ravens by subspecies, they had significantly different and near-infrared absorptance profiles," says Shultz.

"These are birds that just look like they're the same black to us, but their feathers are taking in heat at different rates, so something else is going on."

A better understanding of how color, light, and heat interact with bird feathers could lead to breakthroughs in developing new materials that conduct heat more efficiently, and the same understanding can help us predict how bird populations might cope with rising temperatures. This study is only the first step.

"It's exciting to learn that the feathers of birds are evolving to shed heat into outer space to track climatic challenges," says co-author Dr. Terry McGlynn, Professor of Biology at Cal State University Dominguez Hills.

"We are eager to find out how this works at the microscope scale in bird feathers."

More information

T Lee et al, Population- and species-level variation in near- and mid-infrared radiation in birds: a preliminary analysis, Integrative Organismal Biology (2026). DOI: 10.1093/iob/obag006

Key concepts

electromagnetic reflectance and emissivitybirdsornithologyAdaptation, BiologicalBody Temperature RegulationThermal propertiesInfrared techniques

Provided by Natural History Museum of Los Angeles County