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.