Ecology: Measuring the global gradient of bird colour

Songbirds living in the world’s tropical regions and near the Equator are more colourful than birds living in high-latitude temperate zones, according to an analysis of more than 4,500 species of birds published in Nature Ecology & Evolution. These findings confirm a long-held hypothesis that animals are most colourful at the Equator and become less so as latitude increases.

Eighteenth- and nineteenth-century European naturalists who travelled to the tropics described the rich colourfulness of tropical flora and fauna relative to species in northern latitudes. Although such a gradient has long been suspected, previous studies covered only small geographic areas, measured few species or used subjective measures of colourfulness, such as human scoring.

Christopher Cooney and colleagues made use of improved imaging and colour classification technologies, along with data from the specimen collection at the Natural History Museum in Tring, UK. The authors digitally photographed adult male and female specimens of more than 4,500 species of passerine birds. Then, using a deep learning approach to extract information from photograph pixels, the authors identified the colour (in red, green, blue and ultraviolet values) of the plumage at 1,500 individual points on each specimen. Finally, the authors extracted the total number of ‘colour loci’ per specimen as an intuitive metric of colourfulness.

Looking at broad-scale patterns, the authors found that the number of colour loci is highest at the Equator and decreases with latitude. Colour diversity was highest in birds from closed forest habitats (rather than open areas such as grasslands) and in birds consuming fruits and floral nectar, which may suggest the need for bright visual communication and the ability to acquire colour-forming compounds in the diet as two potential drivers of this pattern.