North American barn swallows, both males and females, with darker breast feathers have better reproductive success. That’s because the feature is recognized as an indicator of health, thus a selector for mating.
While ventral feather color is significantly influenced by genetics, the color of melanin-based plumage, as in barn swallows, is also tied to social status, and to physiological states, such as circulating testosterone and stress level.
A bird whose feathers are artificially darkened to resemble attractive birds rapidly adopts the health of darker birds. Measureable stress level drops.
The same color manipulation induces opposite effects on testosterone in males and females: levels go up in darkened males, and down in spruced-up females; an enhanced health effect in both instances.
Birds made up to look good feel good about themselves. What’s in the mind impacts the body.
Maren N. Vitousek et al, “Female plumage colour influences seasonal oxidative damage and testosterone profiles in a songbird,” Biology Letters (21 August 2013).
Barn swallow photo courtesy of JJ Cadiz.