Evolutionary predictors of the specific colors of birds

Abstract

Animal coloration is one of the most conspicuous aspects of human-perceived organismal diversity, yet also one of the least understood. In particular, explaining why species have specific colors (e.g., blue vs. red) has proven elusive. Here, we quantify for nearly all bird species, the proportion of the body covered by each of 12 human-visible color categories, and test whether existing theory can predict the direction of color evolution. The most common colors are black, white, gray and brown, while the rarest are green, blue, purple, and red. Males have more blue, purple, red, or black, whereas females have more yellow, brown, or gray. Sexual dichromatism is partly due to sexual selection favoring ornamental colors in males but not in females. However, sexual selection also correlated positively with brown in both sexes. Strong social selection favors red and black, colors used in agonistic signaling, with the strongest effects in females. Reduced predation risk selects against cryptic colors (e.g., brown) and favors specific ornamental colors (e.g., black). Nocturnality is mainly associated with brown. The effects of habitat use support the sensory drive theory for camouflage and signaling. Darker colors are more common in species living in wet and cold climates, matching ecogeographical rules. Our study unambiguously supports existing theories of color evolution across an entire class of vertebrates, but much variation remains unexplained.

Drab and distant birds are studied less than their fancy-feathered friends

“…Would the American dipper (Cinclus mexicanus) be less well studied if it had been named the dusky ditch-dipper? In our dataset, birds with common names starting with "black", "gray", "brown", or "plain" (the most common colors of birds; Delhey et al 2023) were studied approximately half as often as all other birds. As committees debate the renaming of birds currently assigned honorific or eponymous common names (Driver and Bond 2021), we encourage them to acknowledge human biases and consider the potential advantages and disadvantages of naming species based on human visual perception.…”

Role of body size and shape in animal camouflage

“…Phylogenetic analyses have similarly indicated no relationship between body weight and countershading in seabirds (Rogalla et al., 2021 ). The prevailing belief is that the evolution of countershading in seabirds is correlated with changes in light environments (Delhey et al., 2023 ; Gomez & Théry, 2007 ).…”

Does fortune follow function? Exploring how consumer preferences drive the functional trait composition of the global songbird trade

“…This finding is unsurprising, as species with carotenoid-rich diets (e.g. frugivores) tend to have more conspicuous colours while species that forage in relatively open habitats often face selective pressure towards drabber or less conspicuous plumage (Delhey et al, 2023;Hill et al, 2002;McGraw, 2005). The relative strength of colour uniqueness as a driver of songbird trade could explain why insectivores were underrepresented in tradethreatened species at a global scale despite being associated with highly attractive song quality and evolutionary distinctiveness; we may expect this pattern of association to differ across or even within different regions, according to cultural differences in the consumer base.…”