Ecological, genetic and geographical divergence explain differences in sunbird (Nectariniidae) colouration

Bird plumages are among the most elaborate ornaments, displaying almost
all colours of the rainbow. Why and how birds are so colourful remains an
open question with multiple and sometimes competing hypotheses. Different
colours in different patches might have different functions and thus
result from different forms of selection (e.g., natural vs. sexual
selection). Here we test three hypotheses that might explain colour
differences: (1) species isolation, (2) light environment, and (3)
Brownian motion. We show that both natural and sexual selection affect the
evolution of sunbird colouration, but that their extent and direction
differs between sexes, by species interactions, and for different patches
across the body. Even though overlap in the light environment explains
part of colour differences in species, no colour metric (brightness and
chroma) correlates to the light environment. It is likely that these
results, where multiple forms of selection influence colouration in
different ways, are more general across birds, highlighting the need to
investigate bird colouration as a network of individual but
inter-connected colour patches.