杏吧原创

It鈥檚 a familiar sight: a flock of birds flying overhead in a classic V-formation, each saving energy by stealing lift from the bird flying ahead. But what鈥檚 in it for the bird out front?

For , it鈥檚 all about taking turns. The leading bird soon swaps places with the bird immediately behind it, in a rare example of a phenomenon called reciprocal altruism.

To understand how birds cooperate in flight, at the University of Oxford and his team tagged every ibis in a group of 14 with high-precision GPS data loggers, allowing them to measure each individual鈥檚 position in relation to the rest of the flock.

Sharing duties

They found that individual birds changed positions frequently, and were only in an aerodynamically helpful position about a third of the time. Most of these formations comprised just two birds sharing duties equally.

鈥淔or whichever combination of two birds we looked at, we saw that the time bird A was flying in front of bird B matched closely the time bird B was flying in front of bird A,鈥� says Voelkl.

And this wasn鈥檛 just an average over the 39 kilometres that the flock flew 鈥� Voelkl鈥檚 team frequently observed swaps within a pair happening within seconds, with the leader moving back behind the same bird for a similarly timed spell of following.

鈥淭his immediacy of the reciprocation reduces the opportunity for cheating,鈥� says Voelkl. 鈥淒irect swaps also mean that you do not have to memorise who is 鈥榦wing鈥� you leading time, so doesn鈥檛 require a lot of memory.鈥�

You scratch my back鈥�

The results strongly suggest that the birds are returning favours in turn, showing what is known as reciprocal altruism. proposed to explain how organisms could help each other without being exploited by cheats. But examples of it have been hard to find and difficult to prove.

鈥淐onvincing examples of reciprocation in animals are rare,鈥� says at the University of Gothenburg in Sweden, who praises the team鈥檚 technique for studying ibis interactions as ingenious.

Until now, vampire bats have provided of reciprocal altruism in animals. When a starving bat has not been able to feed enough to survive the night, other bats regurgitate some of their food for it and share their blood meals.

Although grooming in primates might appear to be another example, this kind of cooperation is often more likely in more closely related individuals, so is in fact associated with kin selection: the giving of favours to individuals that carry a large share of your genes.

In ibises, more than 60 per cent of the aerodynamically beneficial formations formed by the birds consisted of a single pair, with one member flying in front of the other.

Easier for two

鈥淚t is easier for two animals to cooperate together,鈥� says Voelkl. Larger groups of cooperating animals are well known to be more unstable and subject to cheating, he says.

Individual birds often left one pair to form another 鈥� most birds did not spend more than 10 per cent of their time with one specific individual 鈥� but Voelkl鈥檚 team observed that some birds were more likely to pair with certain individuals than others, and that whether two birds were related had no effect on their likelihood of pairing up.

at the University of Giessen in Germany says it is particularly interesting that in the air, birds seem to cooperate in pairings that do not match relationships seen in the birds鈥� behaviour on the ground. He suggests this indicates the birds have evolved a form of cooperation specific to flight.

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