This post continues the hypothetical situation described in my previous post, which you can find here. I left off claiming that the most likely explanation for the barring and display of the male turacos was sexual selection; below I try to support this hypothesis in greater detail:
We know that the male turacos limit themselves to intraspecific displays, so the pigmentation and display must be signals for other turacos. It also seems likely that some element of the barring is, or is at least perceived as, evidence of quality. Depending on how elaborate and visible the display is, it may create a handicap for male turacos. The handicap theory posits that sexually attractive traits are frequently impediments to an organism’s ability to survive, and thus that an organism’s continued survival despite such a hindrance portrays its higher quality. For example, some male birds, peacocks being an immediate example, have displays of quality reflected by an honest handicap: their conspicuous and unwieldy tail feathers. Although the turaco’s ultraviolet pigment is inexpensive to produce and invisible to mammalian predators, it may be noticeable to some reptilian predators and is probably visible to avian predators, making it possible that the turaco barring display is an honest handicap signal.
Since these barred turacos in the Congo have only flashy undersides—and probably solely to birds and snakes, at that—more energetic or longer displays of this conspicuous plumage might be a way for males to signal their fitness via an honest handicap: males who have slightly longer and noticeable displays are exposing themselves to potential predators and proving themselves to females. It is also possible, albeit unlikely, that the turaco’s pigmentation was expensive at some earlier point in its evolutionary history, and that females do not yet “know” that the pigmentation is now inexpensive (vis-à-vis both resources and predation). For instance, predatory snakes may have been more abundant in the past, making conspicuous displays riskier. In such a case, the display would almost certainly have been an honest handicap for the males; if snakes are no longer primary predators today, then displaying a dark cryptic green may not be particularly conspicuous considering the turaco’s dense riparian forest habitat, and thus is not an honest handicap, even if potentially perceived as one. It is also important to point out that such a scenario would depend on the hunting behavior of both snakes and any avian predators.