Individual's condition expressed through visual sexual signals.

This is why people wear make-up, more or less.  This seems like a no-doi experiment.

From Science magazine:
Ecology
Be Honest
Sacha Vignieri
Sexual signals, such as plumage color, are thought to reflect an individual's condition and thus to be a relatively honest indicator of quality to those seeking a mate. The condition of individuals, however, can change over time, leaving one to wonder if such traits only provide honest information about condition at a specific point in the past. Vitousek et al. tested whether signals themselves may influence an individual's condition and thus provide a more accurate indicator of current quality. Specifically, they experimentally darkened the underside of female North American barn swallows and measured indicators of physiological state, such as reactive oxidative metabolites and circulating testosterone. Manipulated birds had consistently lower levels than controls. Naturally darker birds have greater resistance to oxidative stress and reproductively dominate lighter birds. The authors suggest that darkening the birds led to altered social interactions, including fewer challenges and greater mating success. Further, they suggest that the reduced stress experienced by darkened birds left them in better condition, one more reflective of the high-quality trait they displayed. These results suggest that feedback between a signal, its bearer, and recipients may help keep both the signal, and the signaler, honest.
Biol. Lett. 9, 10.1098/rsbl.2013.0539 (2013).