Complex plumages spur rapid color diversification in kingfishers (Aves: Alcedinidae)

Colorful signals in nature provide some of the most stunning examples of rapid phenotypic evolution. Yet, studying color pattern evolution has been historically difficult owing to differences in perceptual ability of humans and analytical challenges with studying how complex color patterns evolve. Island systems can provide a natural laboratory for testing hypotheses about the direction and magnitude (i.e., rate) of phenotypic change. A recent study of bird coloration found that the plumages of island species are darker and less complex than continental species. Whether such shifts in plumage complexity are associated with increased rates of color evolution on islands remains unknown. Here, we use geometric morphometric techniques to test the hypothesis that plumage complexity and island insularity interact to influence color diversity in a species-rich and cosmopolitan clade of colorful birds—kingfishers (Aves: Alcedinidae). In particular, we test three predictions: i) plumage complexity enhances interspecific rates of color evolution, ii) plumage complexity is lower on islands, and iii) rates of plumage color evolution are higher within island systems. Our results show that more complex plumages result in more diverse colors among species and that island species have higher rates of color evolution. Importantly, we found that island species did not have more complex plumages than their continental relatives. Thus, complexity may be a key innovation that facilitates response to relaxed (or divergent) selection pressures on islands. Lack of strong support for competition-driving rates of evolution along different color axes hints at an allopatric model of color evolution in which species adapt to local conditions on different islands. This study demonstrates how a truly multivariate treatment of color data can reveal evolutionary patterns that might otherwise go unnoticed.