Birds are officially more colourful closer to the equator

For anyone that has seen images of bustling tropical rainforests, where you might encounter squawking macaws skimming the treetops and brightly hued frogs singing rhythmically from amongst the leaves, it may seem like common sense to suggest that these are colourful places. But for scientists, the colourfulness of tropical species has been surprisingly difficult to quantify.

Now, after taking some 140,000 pictures of roughly 24,000 birds, the results are in: passerines that live nearer the equator are more colourful than those which live at higher latitudes.

The new research, which used the Museum's extensive collection of birds, came as something of a surprise even to the researchers themselves, who didn't expect to find such a strong association between latitude and colourfulness.

Dr Chris Cooney, from the University of Sheffield and lead author of the study, says, '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.'

The results have been published in the journal Nature Ecology & Evolution.

Why birds living in the tropics are more colourful than their northern and southern counterparts is not so straightforward to answer. Chris and his team have suggested a couple of different reasons, from the dark nature of rainforest interiors meaning it is better to be brightly coloured, to the structural complexity of these environments leading to structural complexity of colour.

But the exact reasons are still unknown and will require further study to tease out the many convoluted interactions that are likely at play.

Rainbow latitudes

When most people think of the tropics, a chaotic environment of jazzy birds, flamboyant flowers and rainbow-tinted reptiles often comes to mind.

This riot of colour was picked up on by many nineteenth century scientists, with Charles Darwin and Alfred Russell Wallace both noting how species in the tropical rainforests that swaddle the equator seem to be more brightly coloured than their traditionally more dour counterparts in northern and southern climes.

Renowned naturalist Alexander von Humboldt, who spent five years travelling through South America between 1799 and 1804, also picked up on this trend, writing about the array of bright-red flowers and the incredible pallet of 'colours in birds, fish, even crayfish (sky blue and yellow)!'.

But while this pattern of colouration has long been noted, it has also been a little contentious.

'For centuries there has been this general idea, this notion that tropical animals and plants are more colourful than organisms that live outside the tropics in cooler climates closer to the polar regions,' explains Chris. 'But although this is a widespread and well ingrained idea, from an empirical perspective, there is not much evidence to back it up.'

It is difficult to study these broad patterns simply because they span huge geographical areas and require massive amounts of accurate data.

Over the years there have been a range of studies both supporting and refuting the idea, largely on the basis that the observations are anecdotal with little scientific scrutiny to back them up. The few scientists who have tried to quantify the distribution of colour in animals and plants around the world have similarly come up with mixed results. These studies have typically used small sample sizes across a range of animal and plant groups.

But by focusing on just a single group of birds - the passerines - this new study has been able to look in far more depth than anyone has before at whether colouration increases the closer species are to the equator. 

How to measure colour

One of the trickiest aspects of conducting any study looking into the colourfulness of animals and plants is the basic question of how to measure colour.

Relatively speaking, it is easy to measure and compare many other aspects of an organism's morphology, for example body size, wingspan, or the length of limbs. But colour is far more abstract. There is not only the range in colour, but the intensity, pattern and distribution.

'This is why it has proven so difficult to quantify,' says Chris. 'There are multiple dimensions of colour.

'We focused specifically on variation in hue and saturation: hue being the type of colour, so red or blue, and saturation being how red or blue it is.'

But this is only one part of the picture. While human and bird visual systems overlap to a large degree, meaning that we generally see the same colours that birds see, birds are also able to see ultraviolet light. This aspect of colour is invisible to us, but likely plays a not insignificant role in the colour perception of many species of birds.

'There are dimensions to bird colour that we are blind to, and so to measure colourfulness in a way that represents the colourfulness from a bird's perspective, we need to measure the colours we can't see,' explains Chris.

This means that not only did the researchers have to assess the colourfulness of birds in the visual spectrum of light, but in the ultraviolet spectrum too. 

The team took six pictures of each bird, making sure they captured all angles of the animals in both visible and ultraviolet light. This led to the taking of some 140,000 pictures of roughly 24,000 bird specimens representing 4,500 species.

The monumental task of photographing the birds took the researchers around three years to complete, but it didn't stop there. The enormous amount of data then had to be processed, a feat that took them another three years.

The fact that this pattern has now been shown for birds means that Chris suspects that it might also hold for other groups, such as flowers, insects and fish, although this remains to be proven.    

'The next steps are to interrogate more finely some of the potential explanations for this pattern,' says Chris. 'These broad associations can point us in the right direction of potential factors that are important in explaining these patterns, but there is so much more to be learnt about precisely what ecological or evolutionary factors are causing this trend.

'We don't know. And at the moment we only have a very broad scale insight into the causes, and I think that is something personally that would be brilliant to follow up on.'