A Female's Prerogative to Change Her Mind?


The come-hither traits of male migratory songbirds on the Colorado prairie -- from flashy plumage to body and beak size -- seem to change from year to year, depending on female preferences, new research suggests. Until now, researchers had generally assumed that factors driving the evolution of male birds’ ornamentation remain fixed over time. But a five-year study of the lark bunting, or Calamospiza melanocorys, turns that notion on its head. Researchers tracked five characteristics of the male birds’ plumage, and three measures of their size: body color, the proportion of black to brown features both on the rump and on the rest of the body, wing patch size, wing patch color, body size, beak size and mass. Then they followed the reproductive success of each male bird, and ran paternity tests to confirm their observations. Male birds' reproductive success seemed to depend on the types of ornamentation in vogue among females that season. Female birds may modify their choices in response to environmental or social changes, Alexis Chaine reports. "If the prairie is overrun by ground snakes, for example, female birds might choose the most protective males -- signaled by, say, wing-patch size," he explains. Female reversals in selection on male traits seem “parallel to oscillating natural selection described in Darwin’s finches,” the researchers note. Follow-up research is bound to look further at how competition among males might have affected male ornamentation, too. This research appears in the 25 January 2008 issue of the journal Science.

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VIDEO is available.

Male lark bunting perched on a bee plant.

[Image courtesy of Alexis Chaine]

Male lark bunting perched on saltbush. Birds were individually identified by color bands, as seen on this male's leg.

[Image courtesy of Alexis Chaine]


Lark bunting nestlings begging for food.

[Image courtesy of Alexis Chaine]

Female lark bunting on a grass nest incubating eggs.

[Image courtesy of Alexis Chaine]



Download video 1. Video of the flight display and song that lark buntings do prior to mate acquisition.

Download video 2. Video of a male and female copulating in the field.

Download video 3. Video of a male singing on his territory. Like the butterfly display, males do this only prior to mate acquistion.

Download video 4. The authors tried various techniques to capture birds for targeted experimental manipulations, and one such methods was to use a taxidermied female in copulation solicitation posture to attract the male. This video shows a male approach and copulate with the female and do a postcopulatory display.

[Video courtesy of Alexis Chaine and Bruce Lyon]



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