Over 40% of male marsh harriers look like females, spending their lives sporting the brown plumage and white heads characteristic of females. They're one of only two bird species that does this. What's the evolutionary advantage of gender bending?
While the males of many bird species will have female plumage as they grow up, marsh harriers are rare in that many males keep this look their entire lives. These female-mimics must possess some reasonably large evolutionary advantage for there to be so many of them, and now researchers in France and Spain say they have the answer. Writing in Biology Letters, they explain what's going on with these unusual birds:
A likely benefit of female mimicry is reduced intrasexual competition, allowing female-like males to access breeding resources while avoiding costly fights with typical territorial males. We tested this hypothesis in a population of marsh harriers Circus aeruginosus in which approximately 40 per cent of sexually mature males exhibit a permanent, i.e. lifelong, female plumage phenotype. Using simulated territorial intrusions, we measured aggressive responses of breeding males towards conspecific decoys of females, female-like males and typical males. We show that aggressive responses varied with both the type of decoys and the type of defending male. Typical males were aggressive towards typical male decoys more than they were towards female-like male decoys; female-like male decoys were attacked at a rate similar to that of female decoys.
Basically, the female-mimics enjoy increased access to females with reduced risk of attack from jealous males. From a reproductive perspective, that's a terrific advantage. But what's really interesting is that this isn't just an act. As far as behaviors go, these female-mimics respond as though they really think they're female:
By contrast, female-like males tolerated male decoys (both typical and female-like) and directed their aggression towards female decoys. Thus, agonistic responses were intrasexual in typical males but intersexual in female-like males, indicating that the latter not only look like females but also behave like them when defending breeding resources. When intrasexual aggression is high, permanent female mimicry is arguably adaptive and could be seen as a permanent ‘non-aggression pact' with other males.