Adaptive and non-adaptive convergent evolution in feather reflectance of Channel Islands songbirds

Convergent evolution is widely regarded as a signature of adaptation. But
testing the adaptive consequences of convergent phenotypes is challenging,
making it difficult to exclude non-adaptive explanations for convergence.
Here, we combined feather reflectance spectra and phenotypic trajectory
analyses with visual and thermoregulatory modeling to test the adaptive
significance of dark plumage in songbirds of the California Channel
Islands. By evolving dark dorsal plumage, island birds are generally less
conspicuous to visual-hunting raptors in the island environment than
mainland birds. Dark dorsal plumage also reduces the energetic demands
associated with maintaining homeothermy in the cool island climate. We
also found an unexpected pattern of convergence, wherein the most
divergent island populations evolved greater reflectance of near-infrared
radiation. However, our heat flux models indicate that elevated
near-infrared reflectance is not adaptive. Analysis of feather
microstructure suggests that island-mainland differences are related to
coloration of feather barbs and barbules rather than their structure. Our
results indicate that adaptive and non-adaptive mechanisms interact to
drive plumage evolution in this system. This study sheds light on the
mechanisms driving the association between dark color and wet, cold
environments across the tree of life, especially in island birds.