How Birds Developed Thicker Down Jackets to Survive the Cold

A recent study of the Himalayan songbirds in museum collections is a step towards using feathers to predict the effects of the climate crisis for birds in extreme environments.

Feathers are sleek, colorful, and intricate evolutionary innovations that make flight possible for birds. In addition, their stiff, aerodynamic feathers keep a layer of fluffy down feathers between their small bodies and the outermost feathers for temperature regulation.

Studying Birds Down Feathers

Sahas Barve, a Peter Buck Fellow at the National Museum of Natural History, Smithsonian, utilized the museum's collection of 625,000 bird specimens. The study published on February 2021 in the journal Ecography, examined feathers across 249 Himalayan songbirds species.

Researchers found that birds residing at higher elevations have more of the fluffy down jacket--the type of feathers humans use to stuff their jackets--than birds from lower areas.

The study also suggests that smaller-bodied birds that lose heat faster than larger counterparts tend to have longer feathers in contrast to their body size, thus a thicker layer of insulation.

Finding such a pattern across a broad number of species underscores the importance of feathered to a bird's ability to adapt to its environment. It suggests that adding down may be a key strategy common to all songbird species, or passerines as researchers call them.

In addition, finding birds from colder environments tend to have more down may help researchers predict how birds, the most vulnerable to the climate crisis, might respond to the challenges of climate change.

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Analyzing Bird Down Feathers

Researchers were inspired by a tiny bird -- goldcrest--during a frigid morning of fieldwork in a forest in the Himalayas. To answer questions raised by the tiny bird, Barve and his colleagues used a microscope to take images of the chest feathers of over 1,700 specimens from the Smithsonian's collection of 249 species from the high-altitude Himalayan Mountains.

Researchers examined the images and measured how long each feather's downy section was relative to the total length. After meticulous analysis, findings show that the smallest birds and birds from high elevations where temperatures were coldest tend to have the highest proportion of down feathers on their body.

The analysis shows that high-elevation birds had up to 25% more down in their feathers, where the smallest birds had three times longer feathers than larger birds.

Barve says, "Seeing this correlation across so many species makes our findings more general, and let us say these results suggest all passerine birds may show this pattern," He adds credit to the Smithsonian's help who without the study would not be possible.

Carla Dove, director of the museum's Feather Identification Lab says "Sahas looked at more than 1,700 specimens. Having them all in one palace in downtown Washington as opposed to having to go to the Himalayas and study these birds in the wild, obviously makes a big difference. It allowed him to gather the data needed quickly before COVID-19 lockdowns swept the globe."

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