Being the leader in a V-formation of migrating birds is hard work, which is why birds in the formation "share the pain" inherent in that role by taking turns, researchers have found.
After a period of leading the flock, the lead bird can save energy by turning the job over to another bird and then moving back in the formation to follow in the wake of another bird, international researchers led by scientists at Oxford University in England have discovered.
The findings are the first conclusive evidence for "turn taking" reciprocal cooperative behavior in birds, the scientists report in the Proceedings of the National Academy of Sciences.
The researchers followed a flock of juvenile Northern bald ibis birds as they migrated from Austria to Italy.
The birds had been raised to be "human imprinted" so they would follow their handlers flying in powered parachutes, with each bird carrying a tiny data logger that allowed researchers to track the position of individual birds within the V-formation.
Individual birds were seen changing position frequently within the flock, they said, dividing their time between leading the formation and taking advantage of the updraft provided by the flapping of another bird's wings, where they can save an estimated 10 to 14 percent of their energy output.
"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," says lead author Bernhard Voelkl of Oxford's Department of Zoology.
"'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."
This pair behavior is also followed in large formations, which can offer even larger energy savings, the researchers say.
"We found that larger formations of ibis were still made up of these 'turn-taking' pairs," Voelkl says. "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."
Long flying migrations present risks for birds, with previous studies suggesting as many as 35 percent of juvenile birds can die from exhaustion in their first migratory flights.
"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," Voelkl says.