Songbirds get more colorful the closer they live to the equator

WCHICKEN DE The Prussian naturalist Alexander von Humboldt arrived in South America in 1799, the colors amazed him. “Look at the blossoms, the birds,” he wrote. “Even the crayfish are blue and yellow.”

In the intervening centuries, Humboldt’s musings have turned into an informal, if controversial, hypothesis about the world’s living things: that organisms in equatorial climates are more colorful than those closer to the poles.

In a new newspaper in Nature Ecology & EvolutionChris Cooney, an evolutionary biologist at the University of Sheffield, and colleagues from the collection at the British Natural History Museum at Tring offer what they call the most comprehensive study of this hypothesis to date, showing that it does indeed apply. is on an order of known birds. as songbirds, a family of songbirds including the blue tit and the robin.

Over several years, Dr. Cooney 4,527 birds from the museum’s archives, representing about half of the total bird diversity on Earth. Because songbirds are sensitive to ultraviolet (UV) are both light and visible, they see more colors than humans. The researchers therefore took photos under UV light, for a real bird’s eye view.

Then a machine learning algorithm was run on the images to identify plumage colors (in values ​​of red, green, blue and ultraviolet) at hundreds of points on each specimen. The total number of colors found on each bird was then mapped to the geographic distribution of their parent species.

The researchers used this data to derive a measure of how colorful different regions of the world were. The main factor here was the diversity of colors on individual birds. The paradise tanager (Tangara chilensis — pictured above), for example, striding through the Amazon rainforest in a green domino mask, an azure bib fading into a sky-blue apron, and a black cape splashed with all the colors of a sunset, is a sign of a colorful region. The monochromatic dunnock (Prunella modular), meanwhile flying across Europe with its brown wings streaked with dark brown, modulated by brown dots scattered across a dark gray face, was unerring evidence of a less colorful area.

Equatorial birds, the researchers concluded, were indeed more colorful than their temperate cousins, with color decreasing with latitude. The birds sampled at the equator averaged between 90 and 100 different colors, while birds at latitude 60 degrees north had closer to 70 colors. Such latitude trends are not uncommon in ecology, where species richness has also been shown to peak at the equator.

Numerous explanations for tropical colorfulness have been offered over the decades. Some have speculated that warmer climates make more resources available to their inhabitants, allowing them to expend energy developing ornamentation. Others have suggested that the bright colors may stem from chemical compounds in their diet, or that breeding pairs may see each other in the dark environment of a tropical forest floor.

The authors of the latest study don’t draw any definitive conclusions on this, but their analysis shows that color is strongly associated with the resources available to birds in an environment, as well as with its diversity. Whether such results can be extrapolated to other families of birds — or even other classes of animals — remains to be seen. After years of photographing birds from different angles, Dr. Cooney hopes that a similar study can be conducted on butterflies. Crucially, they can be made flat.

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This article appeared in the Science & Technology section of the print edition under the heading “Bird Watching”

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