Why Do Flocks of Geese Fly in a “V” Shape?

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In large swaths of the U.S. this time of year, it seems you can’t get more than a few feet without tripping over a Canada Goose; they come down here from the Great White North for the winter months (or, in some places, hang out year-round). When they’re not lounging about in field and stream, you might see them flying overhead, usually in a large "V" formation, with one bird in the lead and the others trailing behind it in two diverging lines.

Why do they fly in a “V”? Not just because a “Q” would be too hard. Scientists have found that the “V” formation serves two functions that make group travel easier.

Give Me A Lift

One reason—first proposed by aerospace engineers Peter Lissaman and Carl Shollenberger in 1970—is that the shape of the formation makes the birds more energetically efficient flyers than they would be flying alone. As a goose flies, air rushes around its wings, creating circular, rotating patterns of air at the ends of the wings called wingtip vortices. The vortices push air downward and upward in different spots (you can see a pretty clear illustration of this here), and if another goose is flying in one of the spots where the air is getting pushed up, it gets some free lift (the air force that directly opposes the goose’s weight) from the efforts of the first goose.

If geese fly in a group and arrange themselves correctly, then every one behind the leader gets a little extra lift and doesn’t have to flap as much to maintain altitude and forward momentum. The less they flap, the more energy they conserve, and the farther they can fly.

For a long time, scientists only had mathematical models, photos and distant observations of live geese to support this idea. The aerodynamics made sense, and observed birds were almost always in positioned in the formation to gain some advantage, but no one was able to directly measure the energetic benefit, if any, to free-flying birds.

Then, in 2001, French scientists had a unique opportunity to do just that when they crossed paths with a film company that had trained great white pelicans to fly in formation behind motorboats and ultralight planes for movie scenes. Researchers from the Centre d'Etudes Biologiques de Chizé, led by ecologist Henri Weimerskirch, fitted the birds with heart rate monitors and then filmed them in flight. Their data showed that the heart rates of pelicans flying in “V” formation were 11.4–14.5 percent lower than that of any one bird flying alone, and that the birds in formation could afford to flap less and glide more, for energy savings of 11.4–14.0 percent.

Given this, why don’t all birds fly in the “V”? It seems the advantages of the formation only apply to larger birds, like geese and pelicans, and aren’t as pronounced for smaller, lighter birds. But other formations may have their benefits, too. While the “cluster” formations that pigeons fly in, for example, actually cost them energy versus flying alone, they might allow large groups to fly close together and maneuver and turn without mid-air collisions.

An Eye On You

Researchers have noticed something funny with some bird flocks: While the birds usually fly in positions that give them some energy benefit, not every bird is always in the expected optimal position, and therefore they get lower energy savings. Some birds just aren’t making the most of the “V,” which got people thinking that there might be another reason for the formation.

That reason might be that the formation allows the birds to maintain visual contact with each other and to communicate, making it easier to keep the group together and navigate. Taking the angles of the formation and what they know about geese’s field of vision and “blind spots” into account, researchers from the University of Rhode Island hypothesize that a “V” angle of 29 degrees or more would allow every bird in the group to see every other bird. There don't appear to be any studies that directly test this idea.

Just like with the energy conservation idea, though, the birds don’t always take up the optimal spot for clear visual contact. Most fly, instead, in positions that give them some benefit in terms of both energy and flock contact, or in one or the other. Those close-but-no-cigar situations beg the question of whether there's another "V" advantage we're not seeing, or if it's just really hard for birds to find and maintain the best spot in the formation.

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November 14, 2012 - 4:30am
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