The great naturalists of the 19th century had already discovered the phenomenon: birds they saw during their journeys to the tropics were more colourful than those they knew from their homeland. What the phenomenon was all about could only be speculated for a long time. Was there a connection between the latitude at which the species lived and their colourfulness? British researchers have now been able to prove, using songbirds, what Darwin and Co. observed in the century before last.
How colourful is colourful?
The thesis that there is a connection between the latitude at which it occurs and the play of colours in the fauna has existed for a long time. Researchers have also tried to prove the connection many times. One of the challenges, however, has always been to measure the colourfulness of birds – this can be done in many different ways. Among other things, objective assessments of colourfulness, i.e. human assessments, have been carried out. However, such assessments are very inaccurate. Thanks to advanced image analysis techniques, researchers have now been able to quantify chromaticity. They examined 24,000 photos of 4,500 specimens of passerine birds from a collection at the Natural History Museum in Tring.
Researchers have long suspected that climatic and ecological conditions at the equator are one reason why passerine birds are so colourful. At the poles or in deserts, environmental conditions are often harsh, forcing birds evolutionarily to have less colour in their plumage. It has also been found that species in wetter, more productive areas are on average more colourful. Furthermore, the forest habitat also contributes to birds being more colourful. Birds that feed on abundant fruits and flower nectar could invest more energy in plumage and their blaze of colour.
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.