An analysis of a photographic dataset for more than 4,500 species of passerine birds shows that male and female birds of tropical passerine species are generally more colorful than their temperate counterparts.
Is life generally more colorful in the tropics?
The possible existence of global-scale trends in organismal colorfulness was suggested by 19th-century naturalists such as Alexander von Humboldt, Charles Darwin and Alfred Russel Wallace, who, upon being afforded the opportunity to travel extensively in the tropics, remarked on the ‘rich variety’ and ‘mixtures of colors’ they encountered during their travels.
Since then, a variety of explanations focused on latitude-associated gradients in biotic and abiotic factors have been proposed to account for the assumed increases in tropical species’ colorfulness, including positive effects of more benign climatic conditions and particular ecological strategies that are more prevalent at low latitudes.
However, in the centuries following these early anecdotal observations, biologists have struggled to conclusively test for the existence of global-scale latitudinal gradients in species colorfulness, calling into question whether this long-assumed biogeographic ‘rule’ really exists at all.
“We tested whether a latitudinal gradient in species’ colorfulness exists for the global radiation of passerine birds (order Passeriformes), the largest avian order, comprising 60% of the 10,000 bird species,” said University of Sheffield’s Dr. Chris Cooney and colleagues.
The researchers analyzed over 140,000 visible and ultraviolet light photographs of male and female museum specimens for 4,527 passerine species (76% of passerine diversity).
They identified the color of the plumage at 1,500 individual points on each specimen by extracting information from photograph pixels.
This then allowed the team to extract the total number of ‘color loci’ per specimen as an intuitive metric of colorfulness.
It’s not entirely clear why tropical birds are more colorful, but the findings suggest that dietary differences between tropical and non-tropical species, as well as the influence of their habitat, could play a key role.
The findings provide insight into how biodiversity is distributed across the planet, and will allow researchers to pinpoint bird ‘hotspots’ of color and become more aware of what could be lost if bird species and their habitats are not effectively conserved.
“This work reveals the broad pattern that bird species tend to be 30% more colorful towards the equator and identifies some general explanations for why this pattern might occur,” Dr. Cooney said.
“This is exciting because it helps us to better understand the factors promoting and maintaining biodiversity at global scales.”
“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 colorfulness in tropical species.”
The team’s findings were published in the journal Nature Ecology & Evolution.
C.R. Cooney et al. Latitudinal gradients in avian colourfulness. Nat Ecol Evol, published online April 4, 2022; doi: 10.1038/s41559-022-01714-1
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