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Songbirds are more colourful the closer they live to the equator

Computer analysis has shown that 19th-century naturalists including Charles Darwin were right: birds near the equator are more colourful

Life 4 April 2022

plumage of Purple Starling (Lamprotornis purpureus)

Close-up of the plumage of a purple starling (Lamprotornis purpureus), which lives across the tropics in Africa

Lara Nouri/University of Sheffield/NHM Tring

Songbirds that dwell in tropical regions closer to the equator are more colourful than those living in milder climates. The findings support the idea that tropical animals are generally more colourful than those that live at high or low latitudes.

The idea that life in the tropics is more colourful was first introduced by 19th-century naturalists such as Charles Darwin and Alexander von Humboldt. Until recently, however, it has been hard to prove this hypothesis due to difficulty in quantifying colouration.

Now, equipped with more advanced image analysis techniques, Chris Cooney at the University of Sheffield, UK, and his colleagues decided to test the hypothesis on songbirds – birds in the group Passeriformes – which comprise around 60 per cent of all bird species.

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The researchers created a deep learning AI program to analyse images of birds based on how colourful their plumage was. They defined colourfulness as the “range of colours that are perceptually different from one another”. So, a highly colourful bird would be one whose plumage had a diverse variety of colours. The team then input images of more than 24,000 individuals representing 4527 songbird species.

After mapping the habitats of each species against its colourfulness, the researchers found that both male and female songbirds that lived close to the equator tended to be more colourful than their temperate counterparts.

They also found that species that live in forests were more colourful than those that don’t. This may be because in a darker, busier forest environment, birds need to be “brighter and showier for attracting others and signalling their identity”, says Cooney.

Colourfulness was also found to be higher in birds that frequently fed on fruits and nectar, which suggests that diet may also contribute to colourfulness.

“Here we’re describing another component of global biodiversity, specifically in relation to colouration,” says Cooney. “Documenting that pattern is really important because it tells us something fundamental about the way that biodiversity is distributed across the planet and helps us to understand more about the processes that generate and maintain it.”

The original hypothesis of greater colourfulness in the tropics wasn’t just limited to birds, though. In his travels, von Humboldt noted that insects and even crayfish seemed to be more colourful near the equator, says Cooney.

“We don’t have much evidence for those other groups yet, but it’ll be really interesting to address the same questions in those groups and see how general the pattern is,” he says.

Journal reference: Nature Ecology & Evolution, DOI: 10.1038/s41559-022-01714-1

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