During their tropical expeditions, the European biologists Charles Darwin, Alexander von Humboldt and Alfred Russel Wallace had described this so colorful wealth of flora and fauna compared to the species of northern latitudes.

"The closer you approach the tropics, the more the variety of structures, the grace of forms and the mixture of colors increase", marveled the German naturalist Humboldt (1769-1859), who notably explored the Amazon jungle.

Since then, several scientists have suspected the existence of a kind of geographical pattern of coloring of species, varying according to latitude, but without being able to provide proof.

To confirm the hypothesis, biologists from the British University of Sheffield studied more than 4,500 species of passerines, the largest family of songbirds (swallows, magpies, larks, nightingales, blackbirds, chickadees, paradise tanagers... .), well distributed throughout the world.

They sifted through the plumages of adult specimens (male and female) from the collections of the Natural History Museum in Tring, England: each was photographed against a black background, from three different angles, with and without a filter UV.

Thanks to a deep learning method (one of the fields of artificial intelligence), they extracted from these photo sessions 1,500 pixels per plumage.

On this basis, they were able to measure a coloration "rate" of each species, then compare it to the location of the bird, with a scoring system.

A Costa Rican hummingbird Dan Kitwood GETTY IMAGES NORTH AMERICA/AFP/Archives

The results confirmed that the closer you got to the equator, the brighter the colors were, and that they faded as you moved away from it.

A kind of "biogeographical rule", globally valid for both sexes, despite slight differences.

What is the cause?

Several explanations have been put forward, including that of Alfred Russel Wallace who saw in the luxuriant vegetation of the tropical forests a natural camouflage allowing the bright colors of the birds to flourish all year round, when their congeners in the temperate forests had to adapt their plumage to bare winter trees.

A prediction that the study confirmed.

"We found that coloration was higher in birds in dense habitats, such as rainforests," Christopher Cooney told AFP.

Another factor advanced: food, since birds feeding on fruits and nectar, more present in tropical latitudes, present more vivid colors, he adds.

His research also establishes a correlation between diversity of species and variety of colors, suggesting that this can help birds "to better distinguish themselves from each other, in an environment teeming with species".

© 2022 AFP