Transitions between colour mechanisms affect speciation dynamics and range distributions of birds

Several ecogeographical "rules" have been proposed to explain
colour variation at broad spatial and phylogenetic scales, but these
rarely consider whether colours are based on pigments or structural
colours. However, mechanism can have profound effects on the function and
evolution of colours. Here, we combine geographic information, climate
data, and colour mechanism at broad phylogenetic (9409 species) and
spatial scales (global) to determine how transitions between pigmentary
and structural colours influence speciation dynamics and range
distributions in birds. Among structurally coloured species, we find that
rapid dispersal into tropical regions drove the accumulation of iridescent
species, whereas the build-up of non-iridescent species in the tropics was
driven by a combination of dispersal and faster in situ evolution in the
tropics. These results could be explained by pleiotropic links between
colouration and dispersal behaviour, or ecological factors influencing
colonisation success. These data elucidate geographic patterns of
colouration at a global scale and provide testable hypotheses for future
work on birds and other animals with structural colours.