Island size predicts the frequency of melanic birds in the color-polymorphic flycatcher Monarcha castaneiventris of the Solomon Islands

Observations that similar traits repeatedly evolve across independent taxa on islands—such as loss of flight in birds or reduction of body size in large mammals—suggest that deterministic processes, rather than drift, drive the convergent evolution of these traits. One such repeated pattern on islands that has received little attention is the evolution of entirely black coloration, or ‘‘melanism.’’ In several taxa, including birds, reptiles, and insects, melanism has evolved on islands despite the proximity of mainland populations with different colors. We explored the evolution of melanism on islands by testing how island size predicts the frequency of melanic birds in Monarcha castaneiventris obscurior, a subspecies of flycatcher from the Solomon Islands that is polymorphic for plumage color (i.e. melanic vs. chestnutbellied). While accounting for potential spatial autocorrelation because some islands are ,1 km apart, we found that island size predicted the frequency of melanic birds, with smaller islands having higher frequencies of melanism than larger islands. As with other traits that have repeatedly evolved on islands, this pattern suggests that melanism on small islands is likely mediated by deterministic processes like natural selection.