Zoologger: The only cross-dressing bird of prey
Species: Circus aeruginosus
Habitat: throughout Europe and western Asia, playing dress-up
Gliding over a bed of reeds in south-west France, a male western marsh harrier circles his nest. Scanning the surrounding area, he spots a second male on a nest just 400 metres away from his own.
Ordinarily this would be the start of a fight. Male marsh harriers are territorial, and don't like another male to set up home within 700 metres of the nest. Yet the new neighbour merits nothing more than a long look.
That's because the interloper is a cross-dresser. Ever since he reached sexual maturity, his feathers have been coloured like a female's. Marsh harriers are one of only two bird species ? and the only bird of prey ? where some of the males mimic females.
Transvestite raptor
Male marsh harriers are mostly grey, with yellow eyes, while females are brown with white heads and shoulders, and brown eyes. Females are also about 30 per cent bigger than males.
Not all males obey the gender rules, though. Up to 40 per cent of them have mainly brown feathers with no greys at all, though they do still have the yellow eyes that mark them as male, and are no bigger than expected for their gender. These female-like males acquire their unusual colour in their second year, and keep it for the rest of their lives.
Males that look like females are common among fish, reptiles and insects, but extremely rare in birds. The only other example is a shorebird called the ruff (Philomachus pugnax). Ruffs breed in groups called leks, and some males mimic females to gain access to real females.
The question is, why do male marsh harriers feel the need to masquerade as females? Audrey Sternalski of the Chiz? Centre for Biological Studies in France and Francois Mougeot of the Arid Zones Research Station in Almer?a, Spain, are trying to find out.
Don't hit me!
Sternalski and Mougeot suspected that female-like males were less likely to be attacked by other males, and thus avoid wasting energy by fighting. They monitored 36 breeding pairs at their nests, and watched how they responded to a series of decoys that looked like typical males, female-like males, and females.
As expected, nesting males were most likely to attack the male decoys, and only attacked female and female-like decoys at the expected low rate. Females rarely attacked any of the decoys, which is normal as it is the males that defend the territory.
So it seems the female-like males can pass for females, at least in the eyes of males, and escape being attacked as a result. But their mimicry runs deeper than feathers. Sternalski and Mougeot say they don't just look like females: they behave like them too.
In their tests, female-like males who were defending nests did attack decoys, but they were more likely to attack female decoys than male ones. That is bizarre, says Mougeot, because from their point of view a female is a potential mate, not a threat. Only a true female should regard another female as a potential rival worth attacking.
A separate field study revealed another unusual twist. Sternalski found that female-like males took an active role in mobbing predators, while normal males did little.
All of which suggests the real winners might be the typical males. Instead of having their territories bordered by aggressive males, they can build a nest surrounded by relatively timid female-like males who pose no threat ? but who can be relied on to drive off the predators.
"There must be some advantage to mimicking females," says Mougeot, otherwise the female-like males would have died out. But he says it's not yet clear exactly what that advantage is.
Journal reference: Biology Letters, DOI: 10.1098/rsbl.2011.0914
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