Disentangling the mechanisms of signal evolution in Tyrannidae flycatchers, part II: plumage elaboration evolved with migration behavior, but is also affected by diet, climate, and drift

Animal coloration is an important communication signal that varies among taxa and affects survival and reproduction. Species-specific color is influenced by a variety of factors including phylogeny, predation, sexual selection, light and resource availability, ecological context, and/or species recognition. Tyrant flycatchers are useful to study color evolution because they are the largest family of birds, occur across broad environmental gradients, and although many genera are monomorphic with drab plumage, some are very colorful. Given the complexity of factors involved, the mechanisms influencing plumage color evolution in Tyrant flycatchers likely involve multiple drivers. In the second of our two-part paper on signal evolution in this avian family, we harnessed a large plumage color database of female and male Tyrannidae species (n = 399) to test the relative importance of geography (climate, latitude), ecology (migration behavior, forest cover, diet), and heterospecific proximity on plumage color evolution. From phylogenetically controlled analyses, we found that female color was largely driven by climate and male color was more exaggerated in migratory species. Sexual dichromatism was also affected by climate and diet, and was more pronounced in migratory species, possibly as a result of color loss in females. Pairwise comparisons between heterospecific color differences and geographic distance were generally weak and consistent with expectations under drift, which contrasted our song results (presented in Part I), but analyses at finer taxonomic scales revealed color may be used for conspecific recognition in some genera. Together with our paper on Tyrannidae song evolution, the color results reflect the importance of testing multiple drivers of signal evolution at different taxonomic scales because flycatcher color variation may have arisen through genetic drift, from a loss of female color in migratory species, and for species recognition in some genera. Future work could formally test the correlation between song and color variation, and use stochastic character mapping to confirm if Tyrannidae dichromatism resulted from female color loss in migratory species.