Fight or flight: birds are not equipped for both

A new study led by the University of Massachusetts Amherst has found a negative link between flight-worthiness and fight-worthiness in birds. According to the scientists, evolutionary pressures demanded that birds could either fly well or arm themselves with bony spurs on their legs – but usually not both. Moreover, developing wings instead of bony spurs seems to be caused by both natural and sexual selection.

“It’s kind of puzzling. Birds have such spectacular songs, plumage and dances, but they mostly don’t have specialized weapons. It’s strange because dancing, singing, fancy feathers, and fighting are all ways of successfully obtaining a mate, and often go together,” said study lead author João C. T. Menezes, a PhD student in Organismic and Evolutionary Biology at UMass Amherst.

By estimating how many species of birds do in fact carry weapons and assessing how much and how well different species fly, the researchers found that, although the vast majority of birds are unarmed, 1.7 percent pack weapons in the form of bony spurs on their legs. However, these species seem to be less proficient in flying than their weapon-less counterparts. According to Menezez, “the best fliers tend to lack spurs, and the most heavily armed fighters tend to struggle in the air.”

To explain this strange phenomenon, the scientists ran a number of simulations and models, which showed that bony spurs may impose heavy evolutionary costs. Although weapons such as plumage, dancing, and the ability to sing helps attracting mates and are thus an advantage in sexual selection, the bony spurs make flying a more energy-intensive activity, decreasing birds’ ability to takeoff easily, and fly fast and far. 

Thus, spurs might make birds more likely to get eaten by predators or require more food to meet their energy requirements. By contrast, their unspurred counterparts can escape easier from predators, eat less, and live to breed again.

“This helps explain why birds have an amazing range of plumage, song and dance, while almost totally lacking in the weaponry department,” Menezes concluded.

The study is published in the journal Ecology Letters.

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By Andrei Ionescu, Earth.com Staff Writer