Birds really are more colorful in the tropics
We've long believed that tropical birds are more colorful than those from more temperate climates. But that assumption has never been proven — until now.
An idea first introduced by Darwin and von Humboldt
Naturalists like Charles Darwin, Alfred Russel Wallace and Alexander von Humboldt noted after traveling the world in the 19th century that nature in the tropics was abnormally vibrant in comparison with their homelands. So began the widespread "rule" that organisms were more colorful in the tropics than in the rest of the world — an idea for which little hard evidence has been found...until today.
A global scale work
The type of studies required to test this "rule" only became possible recently thanks to advances like more cost-effective image processing technologies and more accurate and accessible geographic information for species. These tools helped pave the way for a real global-scale study.
A myriad of colors
Birds are among the most colorful animal groups, with around 10,000 species and a gigantic variety of shades and color combinations. Researchers analyzed the images of more than 4,500 species of passerine birds, like these gouldian finches. Passerine birds like sparrows, jays and crows account for almost 60% of all bird species.
What is colorfulness?
But what does it mean to be more colorful anyways? Well, in this case, it means there is a bigger color diversity within one bird — how many different distinguishable colors it has. This was then turned into a score: The higher, the more colorful.
Mapping the colors
First, researchers divided the entire range of colors into a grid, similar to a chessboard. Then they extracted the pixel colors of pictures of birds' feathers and mapped them to the color grid for each bird. Finally, they counted how many squares on the color chessboard were filled. That's how “colorfulness” was measured.
Tropical rainbow
The results confirmed what Darwin and von Humboldt suspected. Tropical birds from the passerine group were indeed more colorful, around 30% more, than those from non-tropical regions! Possible reasons behind this difference could be due to climate and ecological traits, but it's still not clear.
A rule of thumb?
Alexander von Humboldt was not only baffled by the crazy colors of tropical birds, but also by the colorfulness of other vertebrates, insects and plants. Could this "rule," now confirmed in passerine birds, also be applied to other organisms? It remains to be seen.