Leading the flock can be taxing work, so to share the burden migrating birds take turns, according to a new study.

Whenever you see a flock of birds flying in the sky, they're usually in a V-formation. That's not because of some strange love for the letter, but because this shape allows birds to save energy by flying in the wake of one of their peers. When the leader gets tired, he turns the job over and moves in the back of the formation to catch his breath.

Researchers from Oxford University describe this as reciprocal cooperative behavior, which has only previously been documented in one other animal species: vampire bats.

The scientists reported their findings in the journal Proceedings of the National Academy of Sciences.

To better understand migrating birds' energy-saving tactic, the team followed a flock of juvenile Northern bald ibis (Geronticus eremita) 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.

What they found was that individual birds changed position rather frequently in the flock, taking full advantage of the updraft created by the flapping wings of other birds. In all, members of the group spent 32 percent of their time benefitting from their energy-saving V-formation.

"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," lead author Dr. Bernhard Voelkl of Oxford University's Department of Zoology said in a statement.

"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," he added.

No doubt flying long distances is strenuous work, putting birds at risk. Previous studies have even suggested that 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 explained.

So it seems that for migratory birds, there is a method to their madness.

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