Viewing conditions predict avian plumage contrast

Birds communicate using multiple sensory channels, most commonly through
vocalisations and colourful plumage patches. Such colourful plumage has
evolved through a complex interplay of processes, acting not only on the
ability of a patch to convey information but also in response to
physiological and environmental factors. Although much research on
inter-specific variation in bird plumage has concentrated on sexual
selection, much less work has considered the role of non-sexual selection
and how it is affected by joint effects of avian viewing conditions and
receiver vision. Here we combined taxonomically diverse databases of avian
plumage measurements with habitat use and behavioural data to test the
effect of three factors which effect viewing conditions - habitat
openness, migratory preference, and daily phenology - on avian plumage
contrast, accounting for shared evolutionary history and variation in
avian visual systems. We find that habitat structure and migratory
preference predicted plumage visual contrast, especially for females. Our
study demonstrates the important role of non-sexually selected traits and
viewing conditions in shaping avian plumage contrast.