Birds that live near the Equator are more colourful than birds living further away from the Equator, thought naturalists Charles Darwin (1809-1882) and Alexander von Humboldt (1769-1859). In fact, von Humboldt noted that insects and even aquatic creatures, such as crayfish, seemed to be more colourful nearer the Equator.
The Equator is the imaginary circle at zero degrees latitude that divides the Earth between the Northern Hemisphere and the Southern Hemisphere. It is a line at the centre of the Earth (the ‘waist line’) halfway between the North Pole and the South Pole. Countries on and near the Equator have tropical climates.
The Equator is 40,075 kilometres (24,901 miles) long and goes through 13 countries: 7 countries are in Africa, 3 countries are in South America, and 3 countries are in the Indian and Pacific Oceans.
Africa – Sao Tomé and Principe, Gabon, Republic of the Congo, The Democratic Republic of the Congo, Uganda, Kenya, and Somalia
South America – Ecuador, Colombia, and Brazil
Indian and Pacific Oceans – Maldives, Kiribati, and Indonesia.
Scientists have now found that specifically songbirds living near the Equator are more colourful. The closer to the Equator, the feathers of the bird species are brighter and more colourful, says an article in New Scientist in April 2022.
In 2021, scientist Chris Cooney at the University of Sheffield in the United Kingdom, and his colleagues, wanted to test the initial Darwin and von Humboldt hypothesis on songbirds – passerines from the group of Passeriformes – that comprise about 60% of all bird species in the world.
The scientists, using advanced image analysis techniques, created an Artificial Intelligence (AI) program to analyse bird images based on the colour of their feathers, called plumage.
The scientists defined colourfulness as the ‘range of colours that are perceptually different from one another.’ Therefore, a highly coloured bird would be one whose plumage has a diverse variety of colours (many more colours in comparison with other bird species).
The scientists studied more than 24,000 individual birds, representing 4,527 different songbird species. They mapped the habitats of each species against the colourfulness criteria.
The scientists found that both male and female songbirds that live close to the Equator, a tropical zone, tended to be more colourful than the birds living in temperate zones.
They found that bird species living in forests were more colourful than bird species living away from forests. Chris Cooney and his researchers think that this could be because forests are dark and birds might need to be brighter and showier to attract other birds and signal their identity.
They also found that bird species that primarily ate plant nectar and fruit are more colourful than bird species that do not eat fruit. This suggests that the diet of birds has a link to colourfulness.
Cooney says that documenting these research findings ‘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.’
Cooney says that the researchers don’t have enough evidence about insects and other animal species yet to determine whether all animals are more colourful closer to the Equator, specifically tropical rain forests. Nor whether diet plays a significant role in determining colour in other animal species. Cooney said it would be ‘really interesting to address the same question in those groups and see how general the pattern is.’
Photographer: Martina Nicolls
Martina Nicolls: SIMILAR BUT DIFFERENT IN THE ANIMAL KINGDOM