The long-held idea that birds living near the equator are more colorful than those living closer to the poles is true, suggests a recent study
A recently published study has confirmed an old and widely accepted idea that songbirds living in the tropics are more colorful than their counterparts who live closer to the poles. This idea was independently proposed by English naturalists, Charles Darwin and Alfred Russel Wallace, and by German geographer, Alexander von Humboldt (read more about him here). These three remarkable scientists traveled separately to different destinations throughout the tropics during the early- to mid-1800s, and were all completely enthralled by the vibrant colors they observed there, especially those displayed by birds.
The conjecture that tropical birds are more colorful is an attractive idea but has remained scientifically untested until recently because such a study requires vast amounts of geographic data as well as access to cutting-edge image processing technologies and computer power, of course.
Color and colorfulness
But recently, an international team of scientists based at the University of Sheffield, also with team members at the Natural History Museum at Tring, the University of Pannonia, the University of Bath, and the University of Debrecen, collaborated to test the hypothesis that songbirds are more colorful in the tropics.
Comprising almost 60% of the roughly 10,000 known species of birds alive today, songbirds include such familiar birds as sparrows, finches and crows, are classified as passerines (‘perching birds’), which evolved for perching, based on the arrangement of their toes.
To do this research, the scientists photographed more than 24,000 passerine museum specimens from more than 4,500 songbird species that are curated in natural history museum collections around the world. Most of the specimens used in this study are held by the Natural History Museum at Tring, whose collections encompass representatives from more than 95% of all living bird species.
Photographic pixel data were extracted by ‘deep learning’, an artificial intelligence technique for teaching computers to learn by example (like people do), from 1,500 individual locations on each specimen. Whilst artificial intelligence identified the precise locations to measure, color itself was measured by the researchers, who used several different techniques to account for how birds perceive color. These data were mapped onto a ‘color space’ grid for each bird. This process created a color space database containing more than 36 million unique measures of passerine plumage coloration (Figure 1a).
But what is ‘colorfulness’? Depending upon the study, scientists have defined ‘colorfulness’ in a variety of ways throughout the decades, ranging from how many colors an individual bird’s plumage has to how bright (or saturated) those colors are. For this study, the researchers defined a bird as ‘colorful’ when its plumage showed a large diversity of distinguishable colors within its mapped color space. This definition was then transformed into a numeric score for each species. The higher the number for any given bird species, the more colorful it is.
After the birds were numerically ranked by colorfulness, the researchers charted that value onto a map of where the birds live (Figure 2a).
As you can see in the above maps, songbirds with the largest color scores live in wetter, more productive forested areas — rainforests. This is consistent with previous observations that the lush tropical vegetation provides camouflage throughout the entire year, whereas birds living in temperate regions closer to the poles adapted their plumage so it was less conspicuous when viewed against bare trees in winter.
This study also indicated that frugivorous and nectarivorous songbirds that live in dense forests generally are more colorful, supporting previous research that found links between habitats (ref, ref & ref) and dietary factors (ref).
Further, if you carefully compare the two maps, you’ll notice they show where sexual dichromatism — where males are more colorful than females of the species — is greatest. (Eastern North America is especially striking in this regard.)
I was surprised to learn that larger birds are less colorful than smaller birds, according to this study. But this finding is consistent with previous research that found body size apparently tends to suppress the evolution of colorful plumages, probably due to limits both on the relative number of body feathers and on circulating levels of carotenoids, which are pigments that most birds use to grow yellow, orange and red feathers (more here). But it’s 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 broad pattern that bird species tend to be 30 per cent more colourful towards the equator and identifies some general explanations for why this pattern might occur”, said lead author, Chris Cooney, a research fellow at the University of Sheffield, whose research focuses on making sense of the large-scale patterns of avian biodiversity. This study’s findings neatly confirm what Darwin, Wallace and von Humboldt had originally proposed more than 150 years ago.
“This is exciting because it helps us to better understand the factors promoting and maintaining biodiversity at global scales”, Dr Cooney continued. “However, these broad-scale associations with species’ habitat and dietary differences can only tell us so much and there is much more to be learnt about the precise ecological and evolutionary factors promoting increased colourfulness in tropical species.”
This study also highlights the critical importance of natural history museums for supporting this type of research.
“Research like this is only possible because of the amazing resources at the UK Natural History Museum (NHM), and in other natural history collections around the world”, said study co-author Gavin Thomas, a senior research fellow at the University of Sheffield.
“Broad, global-scale studies of biodiversity such as this 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, might it also apply to other organisms, like reptiles, fishes, insects or even plants?
Christopher R. Cooney, Yichen He, Zoë K. Varley, Lara O. Nouri, Christopher J. A. Moody, Michael D. Jardine, András Liker, Tamás Székely & Gavin H. Thomas (2022). Latitudinal gradients in avian colourfulness, Nature Ecology & Evolution | doi:10.1038/s41559-022-01714-1
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