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It can be mesmerizing to watch a flock of birds soaring through the air, then suddenly the entire flock veers off in a new direction, as if the birds had magically made a group decision. Scientists have long wondered how that happens. So, recently, researchers decided to track individual pigeons in a flock.

As NPR's Nell Greenfieldboyce reports, they outfitted each bird with a GPS device in a little backpack.

NELL GREENFIELDBOYCE: The birds in this study were about a dozen homing pigeons kept by a fancier who lives in Hungary. They looked very cute in their tiny custom made backpacks.

Dr. DORA BIRO (University of Oxford): You have to imagine sort of a backpack where the straps fit around the wings and the kind of compartment fits on the back.

GREENFIELDBOYCE: Dora Biro is a scientist at the University of Oxford, who did this study with colleagues in Hungary. She says, at first, the pigeons would peck at the backpacks.

Dr. BIRO: So, you know, this isn't something that they experience naturally. So they do sort of investigate the backpacks initially. But very quickly they get use to them.

GREENFIELDBOYCE: It helps that the backpacks weren't heavy. Inside was a GPS device that weighed about half an ounce, but it could record a bird's position five times a second. Biro says without this information there'd no way of knowing who in the flock shifted direction first and how the others responded.

Dr. BIRO: The decisions that are made and that spread through the group are made at such short time scales that it's really not visible easily to the naked eye.

GREENFIELDBOYCE: And you can't just assume that the birds out in front are calling the shots.

Dr. BIRO: Birds have a very wide angle of vision, so for them, it is possible that they would be responding to birds who are flying behind them.

GREENFIELDBOYCE: So, to see if pigeons might be following a leader or leaders, the researchers analyzed all of the data logged by the GPS devices during short flights and longer journeys.

Dr. BIRO: I guess from the beginning we weren't sure whether we were going to find any kind of consistent leader-follower relationship. But, in fact, to us surprisingly, we did.

GREENFIELDBOYCE: She says for any given pair of birds...

Dr. BIRO: You can accurately work out which of them is the leader and which one of them is the follower.

GREENFIELDBOYCE: Some birds had more followers than others. This created a hierarchy of influence within the flock.

Dr. BIRO: Basically, every individual gets a kind of a vote in what the flock does, but the weight of your vote depends on your rank, your position in the hierarchy.

GREENFIELDBOYCE: And it turns out, the birds with the most rank tended to fly out in front. The results of this study are reported in the journal Nature. Biro says it's not clear what make a pigeon a great leader in the air - maybe some birds have a better sense of direction or just fly faster.

But the study does show that in these flocks, not all birds were equal. That's what some scientists have assumed when making computer models that use simple rules to mimic the behavior of flocks.

Iain Couzin is a researcher at Princeton University who studies how groups of animals make decisions.

Dr. IAIN COUZIN (Researcher, Princeton University): Individual differences really matter. And that's what people are beginning to realize. And so I think this work sort of paves new ground in that, you know, we're all sort of getting interested in how individual differences really matter and how they relate to collective dynamics.

GREENFIELDBOYCE: He says, while birds are famous for their pecking orders on the ground, this study was able to use new technology to show that something similar happens in flight.

Nell Greenfieldboyce, NPR News.

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