In the hugely popular video game Angry Birds, frustrated victims of egg theft vent their wrath by turning themselves into living bombs and blowing up their piggie tormentors. Of course, the successively more difficult levels of the game make things a little more complicated than that, as the millions who've been hooked know all too well.
Real-life birds don't have quite that kind of firepower, but, as I discovered as I researched my National Geographic book Angry Birds: 50 True Stories of the Fed Up, Feathered, and Furious, they've evolved an amazing array of ways to display their ire. Mockingbirds dive-bombing intruders, bellbirds ringing their nests with paralyzed poisonous caterpillars, eagles attacking hang gliders, frigatebirds pirating food from weaker birds—the variations of avian aggression seem endless.
And you probably don't even want to hear about baby fulmars, who projectile-vomit oily gunk to defend themselves against predators. (What's that bird? See National Geographic's Backyard Birding guide.)
Keep Your Eye on the Sparrow's Wing
Birds don't have to be big and powerful to show anger. Even sparrows—small, shy, brownish birds that tend to skulk in the underbrush—turn into mini-Hulks when breeding season comes around.
The swamp sparrow, a species found in wetlands through much of eastern North America, performs an odd display called the wing wave. Raising and quivering one wing at a time, the sparrow seems to be bidding good-bye to a friend, or maybe trying to dry its deodorant.
A team of scientists led by Duke University's Rindy Anderson, a specialist in animal communication, decided to look into the swamp sparrow's wing wave. "On a basic level, we wanted to know: Is wing-waving a communications signal?" Anderson said. "And if so, what information is being communicated? Our hunch was that wing-waving behavior, which is often paired with singing of various kinds, was playing some role in male-male aggressive signaling." (Find out why animals are smarter than you think.)
In simple terms, wing-waving may be the bird's way of saying, "Bring it on, buddy"—the equivalent of a stare-down in a biker bar.
Enter the Robo-bird
Many birds use song to claim a nesting territory, but it was difficult to test whether wing-waving was truly part of the male sparrow's homeland defense. As reported in the journal Behavioral Ecology and Sociobiology, Anderson's team came up with a clever experiment. They put a tiny mechanism inside a stuffed swamp sparrow so the taxidermied bird could mimic the wing wave, then placed the "robo-bird" in territories of nesting swamp sparrows. While playing a recorded song, they watched real birds' responses to the robo-bird in three modes: motionless, turning from side to side (to test whether simple movement had an affect), and wing-waving.
"It turned out the real birds were significantly more aggressive to the wing-waving robot than they were to either the motionless robot or the turning robot," Anderson said. "The intruder's wing wave might be signaling aggressive intentions, in the sense that it's signaling, 'I am here, and I am prepared to fight.' And the other bird then has to decide what they're going to do about it."
The Studs and the Wimps
Anderson found that individual birds varied in how aggressively they approached and attacked the robot. "You have wimps and you have studs and everybody in between," she says. She was surprised, though, that wing-waving reaction to the intruder didn't vary for individual birds; each male waved its wings a certain number of times that remained consistent no matter the robo-bird's behavior.
"I expected that there would be more wing-waving by the live birds in response to the wing-waving robot than in response to the other robots, but that didn't happen," Anderson says. "The birds were consistent in their own signaling behavior regardless of what the intruder was doing." Because actual fights can cause injury or even death, each male seems to have its own level of attempted visual intimidation through wing-waving before attacking a rival. No matter the provocation, studs stay studly and wimps stay wimpy.
There's still more to learn about the swamp sparrow's wave, Anderson says. "Because wing-waves and song are almost always given together, we're still not clear about whether those two displays are redundant. If a bird sings and wing-waves at the same time, is it signaling the same thing by the two behaviors, or does the wing-waving signal something different?"