The link between colorfulness and extinction risk across the world's passerine birds

Many of the functional traits that mediate extinction risk across the range of biodiversity relate indirectly to a species' ability to persist in a changing world, yet certain traits such as coloration directly affect human interactions with wildlife. We used an existing dataset of color metrics combined with global functional trait data for 4334 passerine bird species to determine whether colorfulness-that is, the total number of discrete colors in trichromatic color space-predicts two measures of extinction risk: a binary variable indicating whether a species is least concern or not and an ordinal variable of threat level. We also controlled for the potentially confounding effects of four other traits: body size, dispersal ability, forest dependency, and diet. In addition, we assessed whether the global relationship between extinction risk and colorfulness varies by biogeographic realm, breeding latitude, or human development index. Overall, we found that compared with drab species, colorful passerines are at a greater risk of extinction, and this is especially true in Indomalaya and Australasia. We also found that the strength of this relationship is greater for species at higher breeding latitudes and species inhabiting countries with a lower human development index. Finally, we showed that more colorful birds are at greater risk of extinction due in part to the pet bird trade, which favors more attractive species; however, the positive relationship between extinction risk and colorfulness exists even for species not known from the pet trade, suggesting other mechanisms for this relationship. Closing the gap in threat status requires accurate evaluation of the threats to drab and colorful species alike, while also combatting the known threats to colorful birds including illegal trade.