Constraint and innovation in color evolution among species and among plumage patches in five avian radiations

Understanding the causes and limits of phenotypic diversification remains a key challenge in evolutionary biology. Color patterns are some of the most diverse phenotypes in nature. In birds, recent work within families has suggested that plumage complexity might be a key innovation driving color diversity. Whether these patterns hold at larger taxonomic scales remains unknown. Here, we assemble a large database of UV-Vis spectral data across five diverse clades of birds (45791 spectra, 1135 species). Using multivariate phylogenetic comparative methods, we compare evolutionary rates and color space occupancy (i.e., quantification of observed colors) among these clades. Novel color-producing mechanisms have enabled clades to occupy new regions of color space, but using more coloration mechanisms did not result in overall more color space occupancy. Instead, the use of more color-producing mechanisms resulted in faster rates of color evolution and less integrated color among plumage regions. Flexible Bayesian modeling further allowed us to assess the relationship between interpatch and interspecific directions of color variation. We find that interpatch variation generally diverges from interspecies cladewise trends in males but not females, suggesting developmental or selective constraints operating in females across evolutionary scales. By comparing rates among clades and assessing both interpatch and interspecies color variation, we reveal how innovations and constraints operate across evolutionary and developmental scales.