Birds that live near the equator are more colourful than those closer to the poles, a new study suggests.

The findings prove a long-held theory that was first suspected by Charles Darwin and Alexander von Humboldt in the 18th and 19th centuries.

It had remained unproven until now because of the large amount of data and advanced imaging technology needed.

But the study found that tropical birds near the Equator are around 30% more colourful than non-tropical birds that live closer to the north and south poles.

Dr Chris Cooney, lead investigator from the University of Sheffield’s school of biosciences, said: “This work reveals the broad pattern that bird species tend to be 30% more colourful towards the equator and identifies some general explanations for why this pattern might occur.

“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 colourfulness in tropical species.”

Researchers photographed more than 4,500 species of passerine, or perching, birds such as sparrows, songbirds and finches and used this information to identify the colours of feathers.

While it is not entirely clear why tropical birds are more colourful, 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 researchers photographed more than 24,000 birds from the Natural History Museum collection in Tring, which has specimens of more than 95% of the world’s living bird species.

The findings are published in Nature Ecology Evolution.