The long-held idea that birds living near the equator are more colorful than those living near the poles is true, a recent study suggests
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A recently published study has confirmed an old and widely accepted idea that songbirds living in the tropics are more colorful than their counterparts that live closer to the poles. This idea was independently proposed by English naturalists, Charles Darwin and Alfred Russel Wallace, and German geographer, Alexander von Humboldt (read more about him here). These three remarkable scientists traveled separately to different destinations throughout the tropics in the early to mid-1800s and were all completely mesmerized by the vibrant colors they observed there, especially those displayed by birds.
The conjecture that tropical birds are more colorful is an enticing idea but has not been scientifically tested until recently, as such a study requires large amounts of geographical data as well as access to data processing technologies. state-of-the-art image and computing power, of course.
Color and colorfulness
But recently, an international team of scientists based at the University of Sheffield, also with team members from Tring Museum of Natural History, University of Pannonia, University of Bath and University of Debrecen, collaborated to test the hypothesis that songbirds are more colorful in the tropics.
Comprising nearly 60% of the approximately 10,000 known species of birds alive today, songbirds include familiar birds such as sparrows, finches and crows, are classified as passerines (“perching birds”), which have evolved for perching, based on the arrangement of their toes.
To do this research, the scientists photographed more than 24,000 passerine museum specimens of more than 4,500 species of songbirds that are held in the collections of natural history museums around the world. Most of the specimens used in this study are held by the Tring Museum of Natural History, whose collections encompass representatives of over 95% of all living bird species.
Photographic pixel data was extracted by “deep learning”, an artificial intelligence technique for teaching computers to learn by example (as people do), from 1,500 individual locations on each specimen. While artificial intelligence identified the precise locations to measure, the color itself was measured by the researchers, who used several different techniques to explain how birds perceive color. This data was mapped onto a “color space” grid for each bird. This process created a color space database containing over 36 million unique measurements of passerine plumage coloration (Figure 1a).
But what is “color”? According to the study, scientists have defined “colority” in various ways over the decades, ranging from the number of colors in a bird’s plumage to the brightness (or saturation) of those colors. For this study, the researchers defined a bird as “colorful” when its plumage exhibited a high diversity of distinct colors in its mapped color space. This definition was then transformed into a numerical score for each species. The higher the number for a given species of bird, the more colorful it is.
After the birds were sorted numerically by color, the researchers plotted that value on a map showing where the birds live (Figure 2a).
As you can see from the maps above, the songbirds with the highest color scores live in wetter and more productive forest areas – the rainforests. This is consistent with previous observations that lush tropical vegetation provides year-round camouflage, while birds living in temperate regions closer to the poles have adapted their plumage so that it is less conspicuous when exposed. he is seen against bare trees in winter.
This study also indicated that frugivorous and nectarivorous songbirds that live in dense forests are generally more colorful, supporting previous research that has found links between habitats (ref, ref, and ref) and dietary factors (ref).
Also, if you compare the two maps carefully, you’ll notice that they show where the sex dichromatism – where the males are more colorful than the females of the species – is highest. (Eastern North America is particularly striking in this regard.)
I was surprised to learn that large birds are less colorful than small birds, according to this study. But this finding is consistent with previous research which found that body size apparently tends to suppress the evolution of colorful plumages, likely due to limits on both the relative number of body feathers and the circulating levels of carotenoids, which are pigments that most birds use to grow. yellow, orange and red feathers (more here). But it is also possible that visual communication differs by size, with smaller bird species interacting at closer viewing distances and in denser habitats that are advantageous for colorful plumages (ref).
“This work reveals the general pattern that bird species tend to be 30% more colorful towards the equator and identifies some general explanations why this pattern might be occurring,” said lead author Chris Cooney. researcher at the University of Sheffield, whose research focuses on understanding large-scale patterns of avian biodiversity. The results of this study fully confirm what Darwin, Wallace and von Humboldt originally proposed over 150 years ago.
“It’s exciting because it helps us better understand the factors that promote and maintain biodiversity on a global scale,” Dr Cooney continued. “However, these large-scale associations with species habitat and dietary differences can only tell us so much and much remains to be learned about the precise ecological and evolutionary factors promoting increased color in tropical species. .”
This study also highlights the crucial importance of natural history museums in supporting this type of research.
“Research like this is only possible because of the incredible resources of the UK’s Natural History Museum (NHM) and other natural history collections around the world,” said the co. -study author, Gavin Thomas, senior researcher at the University of Sheffield. .
“Such global biodiversity studies are facilitated by the fantastic work of museum curators and volunteers, and museum collections continue to provide the raw material for cutting-edge scientific research.”
Now that this hypothesis has been confirmed in birds, could it also apply to other organisms, such as reptiles, fish, insects or even plants?
Christopher R. Cooney, Yichen He, Zoë K. Varley, Lara O. Nouri, Christopher JA Moody, Michael D. Jardine, András Liker, Tamás Székely and Gavin H. Thomas (2022). Latitudinal gradients in avian coloration, Nature ecology and evolution | doi:10.1038/s41559-022-01714-1
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