- A new study has confirmed what biologists have long suspected: that tropical birds are much more colorful than their temperate peers.
- The study involved a mathematical color analysis of more than 24,000 pictures of 4,500 birds from around the world, and found that species from Amazon, West Africa and Southeast Asia were on average 30% more colorful than those in the Northern Hemisphere.
- The researchers suggest this difference may be down to energy availability: with more food year-round and more constant temperatures, tropical birds have been able to evolve more elaborate visual signals.
- Diet is another possible factor: fruits rich in organic pigments are abundant in the tropics, and this pigment tends to accumulate in the feathers of the birds that eat them.
“The closer one gets to the tropics, the greater the variety of structures, graceful forms and color combinations.” Such were the words of German naturalist, geographer and explorer Alexander von Humboldt (1769-1859), one of the founders of the school of biogeography.
During his extensive travels in the Americas in the early 19th century, Humboldt noticed — concurrently with other household names from the field, including Alfred Russel Wallace and Charles Darwin — that both plants and animals were more colorful in regions near the equator.
Today, more than a century and a half later, a study analyzing high-resolution digital imagery of 4,500 passerine, or perching, birds from around the globe confirms what Humboldt suspected all those years ago: that birds really do come in a much broader range of colors in the tropics. The study also goes on to identify likely reasons for this clear relationship between colorfulness and latitude.
On average, the study found that birds inhabiting forests in countries like Brazil, New Zealand, Indonesia, Australia, Ecuador and Chile, among others, are 30% more colorful — as measured by reflectance of their plumage — than those found in the Northern Hemisphere.
According to the study, the most colorful birds live mostly in dense and humid forests in the Amazon, West Africa and Southeast Asia.
The study, by biologists in the U.K. and Hungary, uses mathematically based color spectrum quantification techniques to show that the birds in the hottest regions of the globe have more vibrant feathers than their peers in temperate zones.
And not only are their colors more intense, but the spectrum of tones is broader for both males and females.
“Even though it had been a hypothesis for a long time, direct evidence at this level was surprisingly scarce until now,” said lead author Christopher Cooney, from the University of Sheffield. U.K.
He said previous studies used a human point system to quantify species’ colorfulness, or color information derived from field guide or manual illustrations. “While informative, both approaches have their disadvantages because they are not direct measures of the birds’ colorfulness,” he said.
“We aimed to improve this analysis by using digital cameras modified to quantify the birds’ colorfulness, probably in the way the birds themselves perceive it, because we measured the colors in both visible and ultraviolet light (which birds can see),” Cooney added. “This way, we could present much better analysis of the birds’ coloring and prove the relationship between latitude and colorfulness.”
A new way of measuring birds’ color
The researchers analyzed only the order of passerine birds, distinguished by their foot configuration that allows them to perch on a branch. They account for 60% of all known bird species, including the small birds most of us are most familiar with, such as sparrows, canaries and finches.
The analysis process was extremely complex. Photos were taken from three different angles (back, side and front) of each specimen, totaling some 24,000 images. Then, pixel values were extracted from each photo and mapped in tetrahedral avian colors. Finally, more than 36 million unique feather coloring measurements were taken of these birds.
“Evaluating color has always been very difficult. Until now, it was done in a very subjective manner,” said José Alexandre Felizola Diniz Filho, a professor of evolutionary ecology at Goiás Federal University in Brazil who was one of the reviewers of the study.
“The study quantifies not only how many colors each bird has, but also how much variation there is in each color spectrum. So we could calculate indices for how colorful each bird is,” he added.
But the researchers still don’t know with any certainty why birds from hotter regions have a broader range of colors. They have some hypotheses, the strongest of which is related to the concept of energy availability.
“These environments are typically hot, humid and have high net primary productivity, which is a measure of how much energy is available to those living there,” Cooney said.
“We believe that having increased available energy allows species to invest more energy in elaborate visual signals than those species living in colder environments with less available energy.”
Energy is also related to metabolism and evolution. “Their metabolism has to be higher in order to have more colorful feathers,” Diniz Filho said. “They need to have greater evolution of cellular structures that generate more color. For that, they need more energy.”
He said that, in general, male birds in thick forests have more brightly colored feathers to attract females. This strategy increases their chances of mating and, again, is advantageous from an evolutionary standpoint.
Abundance of fruit may contribute to vivid colors
Another factor that differentiates tropical birds’ habitats from those found farther from the equator is climate stability. With more constant temperatures year-round, these birds have more opportunities to mate and more access to food — and therefore, to energy.
In fact, the study also suggests that the fruit-rich diet in tropical zones would also influence animal coloring. Places like the Amazon have an abundance of fruits like mango, acerola cherry (Malpighia emarginata), tucumã (Astrocaryum aculeatum) and umari (Poraqueiba sericea), all rich in carotenoids, a group of yellow, orange and red organic pigments.
Those very colors found in in birds, Cooney said, “are typically produced by carotenoid pigments deposited inside them. The birds have no way of synthesizing them on their own so these compounds must be ingested as part of their diets.”
“Fruits are known to contain relatively high concentrations of these substances, therefore it is probable that fruit-eating species may have more of these compounds available in their diet to produce brilliant colors than species who eat other foods,” Cooney said.
While the study clears up one of the greatest lingering suspicions in biology, Cooney said much remains unknown on this particular issue. There’s some doubt, for instance, whether this pattern is restricted only to birds, or also applies to flowers, insects and fish.
“I would say that similar tendencies may occur in other organisms, but they haven’t been proven yet,” Cooney said. “There is still much more to learn about the ecological and evolutionary factors that drive increased colorfulness in tropical species.”
Cooney, C. R., He, Y., Varley, Z. K., Nouri, L. O., Moody, C. J., Jardine, M. D., … Thomas, G. H. (2022). Latitudinal gradients in avian colourfulness. Nature Ecology & Evolution, 6(5), 622-629. doi:10.1038/s41559-022-01714-1