Bird plumage patterns by geography

Amazing stuff!

"Birds have a remarkable variety of plumage colors that vary across space. Using digitized illustrations from the Handbook of the Birds of the World, Han et al. investigated how abiotic and biotic factors influence this striking variation. They found support for two long-standing hypotheses linking plumage to climate: Birds are darker in both warm humid and cold locations, where darker coloration may reduce parasite loads and increase radiation absorption, respectively. Although plumage patterning is constrained by body size, the spectrum from blue to yellow coloration likely depends on diet, because yellow and orange carotenoid pigments are more available from food sources in the tropics. This work shows how multiple factors intersect to shape the geography of plumage coloration in birds."

From the abstract:

"Animal coloration exhibits substantial biogeographic variation, reflecting complex adaptations to both abiotic and biotic environments. Although the evolutionary mechanisms underlying avian plumage coloration have been widely studied, geographic variation in plumage patterning has received far less attention.

To extend our understanding of avian plumage evolution, we quantified global plumage coloration and patterning across 10 290 bird species using an extensive red-green-blue (RGB) dataset derived from illustrations in the Handbook of the Birds of the World. To test ecogeographical predictions, we modeled variation in plumage lightness, blue–yellow chromaticity, and pattern granularity using phylogenetic mixed models and path analyses. Our analyses revealed pronounced geographic heterogeneity in both plumage coloration and patterning.

Variation in plumage lightness followed broad climatic gradients, and darker coloration was associated with humid and colder environments, consistent with the combined predictions of Gloger's rule (darker in warm, humid areas) and Bogert's rule (darker in cold areas).

These rules are not contradictory but coexist; their relative influence is determined by whether humidity or temperature is the dominant selective force in a given region. While resident species in cold environments largely supported Bogert's rule for thermoregulation, migratory species and proficient fliers (e.g. gulls) deviated from this pattern by maintaining lighter plumage to avoid overheating during flight. Furthermore, we identified a strong latitudinal gradient along the blue–yellow axis: tropical humidity promotes carotenoid-rich yellow plumage through dietary shifts, whereas higher latitudes favor structural blue hues. Patterning also showed large-scale structure, with larger pattern elements linked to humid environments and larger body sizes rather than to specific foraging habitats. By integrating classical ecological rules with the thermoregulation hypothesis, our study provides a unified framework explaining how climatic constraints and ecological processes interact to shape the global distribution of avian coloration patterns."

In Other Journals | Science

Global distribution of avian plumage coloration and patterning: the relative influence of climate, ecology, and sexual selection (open access)

Fig. 2 Principal component analysis (PCA) of species-averaged plumage values (lightness, color, and patterning) for 10290 bird species.

Fig. 3 Latitudinal and global geographic variation in avian plumage coloration and patterning with a 95% confidence interval (CI) during the breeding and non-breeding seasons.