Why some birds of prey become transvestites

From MSNBC:

Birds of prey may be thought of as fierce foes, but scientists find that some males disguise themselves as peaceful females. These males belong to a species of raptor known as the marsh harrier. Using plastic decoys, French researchers learned that the transvestites among these predators are less aggressive than other males. Some animals will use the tactic known as sexual mimicry in the cutthroat battle to survive. For instance, young male birds often have female plumage that helps camouflage them; they will acquire more striking plumage only after reaching sexual maturity, to help them attract mates. However, permanent lifelong female mimicry, in which males look like females throughout life, is extraordinarily rare in birds. Until now, it had been studied in only one species, the ruff (Philomachus pugnax), a shorebird in which some males engage in female behavior to sneakily get sex.

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The only other bird in which this practice has been found is the marsh harrier (Circus aeruginosus). In one exceptional population in midwestern France, 40 percent of the males of this bird of prey disguise themselves in female plumage. To study the marsh harriers there, ornithologists including Vincent Bretagnolle, directorof the Center for Biological Studies of Chizé, France, created decoys painted to closely resemble the females, typical males, and female-like males. Females are mostly brown with ocher-brown eyes, while typical males are mostly gray with yellow-white eyes, and female-like males are mostly brown with yellow-white eyes. Males are also approximately 30 percent smaller and lighter than females. The researchers then spent three months watching how both kinds of males responded to decoys placed in the wild near the nests of 36 breeding pairs of marsh harriers. Some funny situations arose during this field work.

More here.