A new study finds that male sparrows use wing waves as an aggressive signal to avoid a dangerous brawl.

Biologist Rindy Anderson, from Duke University, and her colleagues have revealed that male birds show aggressive behavior by flipping their wings in order to defend their territory and their mates from other males. "It's high stakes for these little birds. They only live a couple of years, and most only breed once a year, so owning a territory and having a female is high currency," Anderson said in a statement.

To understand the birds' wing-waving behavior, the research team used a robosparrow that looked just like a male swamp sparrow and flipped its wings like a live male.

The robosparrow was taken to a breeding ground in Pennsylvania and was placed in the territories of live male swamp sparrows. Using a nearby sound system, the robotic bird "sang" swamp sparrow songs to garner attention from live males.

Apart from the robosparrow, the researchers placed a stuffed sparrow that remained stationary and one that twisted from side to side to test the aggressive wing-waving behavior of the live males. They found the wing waves combined with song are more potent than song on its own. In particular, the wing waves evoked aggression from live birds.

Surprisingly, researchers also noticed that the live birds gave strikingly similar aggressive wing-wave signals to all the three invaders. If a live male sparrow waved its wings five times to the stuffed sparrow, he would also wing-wave five times to the wing-waving robot, said the researchers.

Moreover, Anderson had thought that the defending birds would match the signals of the intruding robots. But the level of aggression that the males wanted to signal was individualistic and consistent. "That response makes sense, in retrospect, since attacks can be devastating," Anderson said.

The findings of the study appear in the journal Behavioral Ecology and Sociobiology.

Click here to see how live angry birds flip their wings to show aggressive signals.