Being the front bird of an iconic V-shaped formation might seem like a prestigious and prominent position, but it's actually an energetically costly one. According to a study published in Proceedings of the National Academy of Sciences this week, birds flying in a V-flock take turns being in the lead. These findings might just be the first solid evidence for "turn taking" reciprocal cooperation in birds.
The northern bald ibis (Geronticus eremita) is a large, critically endangered migratory bird. A 2014 Nature study revealed how these ibises strategically position themselves to take advantage of the updraft created by the preceding bird’s flapping wings. Being in sync with this “aerodynamic up-wash” saves them energy during their long flights. With a third of juvenile birds dying from exhaustion on their first migration, that extra lift is literally a lifesaver. However, the bird at the head of the formation receives no aerodynamic benefits. This, of course, presents an awkward social dilemma: Who would ever want to fly in front?
To see why this potentially altruistic behavior would persist, Oxford’s Bernhard Voelkl and colleagues tracked 14 juvenile northern bald ibises, who were raised by humans at the Salzburg Zoo in Austria. The team attached a 23-gram data logger to each bird—to record their geographic location, velocity, and position within the flock—as they learned their 1,353-kilometer (841 mile) migratory route by following a paraplane from Salzburg to Orbetello, Italy, during August and September of 2011.
The birds, they found, changed positions frequently within their flocks of two to 12 birds. Individuals spent 32 percent of their time, on average, benefitting from the updraft and then a proportional amount of time leading, regardless of genetic relationship. And they switched positions so often and quickly, Science reports, that the benefits of cooperating were immediate.
“Our study shows that the 'building blocks' of reciprocal cooperative behavior can be very simple: ibis often travel in pairs, with one bird leading and a 'wingman' benefiting by following in the leader's updraft,” Voelkl says in a news release. “We found that in these pairs, individuals take turns, precisely matching the amount of time they spend in the energy-sapping lead position and the energy-saving following position.” And members of these pairs would change, New Scientist reports: Most birds didn’t spend more than 10 percent of their time with one specific individual.
So, no freeloaders then? Surprisingly, they found no evidence of cheating. Even the larger ibis formations were made up of these turn-taking pairs. “The checking that went on within these pairs was sufficient on its own to prevent any freeloaders hitching a free ride within a V-formation without leading,” Voelkl explains. “We think that it is the extreme risks associated with long migration journeys that have driven the evolution of such cooperative behavior where something like saving 10 percent of your energy can make the difference between life and death.”
Here are the human foster parents, some hand-reared ibises, and the ultralight powered parachute used to lead the birds on their migratory flight path:
Images: Markus Unsöld (top), Oxford University (middle), Johannes Fritz (bottom)