This post continues the hypothetical situation described in my previous posts, the first of which you can find here.
What if the turaco barring and display were a different sort of quality indicator? They could be amplifiers that emphasize a trait perceived by other turacos as directly related to high fitness. For example, some birds’ feathers, such as those of the European jay, are colored in stripes that develop similarly to, or perhaps accentuate, growth bars. As the turaco’s barring correlates directly to its age, the bars must have typical widths at certain ages. These widths, along with the display, probably serve as a standard by which females can compare males, and males assess competitors. The bars are likely indicators similar to the stripes on jays and other birds—that is, they may portray the bird’s growth regularity, or accentuate flight behavior, body size, feather wear, symmetry, and other potential quality indicators. Should the turaco barring amplify any of these possible traits, then displaying the bars is not a deceptive signal, because it genuinely reflects quality. For instance, if a three-year-old male turaco has successfully avoided predators and found the best food throughout his life, his barring should reflect such facts, and portray his success through low feather wear, larger body size, or more regular bars; a one-year-old male with similar traits but different barring would presumably be less preferable to females. The costs of a certain trait are the mechanism whereby turacos ensure that mutant barring cannot be deceiving: even if a male turaco develops widths of bars differently than his age would predict (i.e. those of a typically older bird), the bars may be imperfectly formed or less vivid, because the turaco’s flight ability or display may not match what would be expected from his bars; or the feathers may show increased wear from the bird being forced to forage in faraway or low quality areas. Since these turacos live in a dense colony, there is a high likelihood that even minor discrepancies between cues and signals would become quickly apparent to male competitors and potential mates.
Depending on the degree of variation in male barring, it is also possible that bars could help distinguish individual turacos in the colony. Although this possibility seems very unlikely given current knowledge, individual recognition could help identify kin or mates, emphasize hierarchies, and discriminate between neighbors and strangers in a colony. Turaco barring probably does not have a significant effect on individual recognition, however, because the characters selected for that would aid individual recognition are quite often different from characters selected for indicating quality.