Why do geese fly in a V?

Geese most likely fly in a V to save energy: that is the usual explanation, and measurements on birds back it up. A wing that makes lift pays a price, called lift-induced drag, and that energy ends up mainly in swirling air at the wingtips. The leader's wingtips leave a pair of spinning vortices: behind the bird the air is pushed down, but just outside it the air is pushed up. A following bird that sits in that rising air can, hypothetically, get a free lift. Geese in a V may conserve 12 to 20 percent of the energy they would need to fly alone.

Some of the clearest evidence comes from the birds' own bodies. In a study of great white pelicans, those flying behind the leader needed fewer wing flaps to stay aloft, and their heart rates were lower. Geese also seem to share the work. On longer flights the leading geese switch positions so that several individuals get the cheaper trailing spots, and in family groups the parents almost always lead.

There may be a second benefit: the formation helps geese communicate and keep an eye on their flock mates.

The V is not a perfect, proven machine. A famous 1970 study claimed that each bird in a V of 25 could extend its range by 71 percent, but that was the authors' claim, and the estimate for geese is far more modest, at 12 to 20 percent.

Even so, birds seem to fine-tune their position. Northern bald ibises in a V keep their wing beats in step so they catch the upwash through the whole flap, while birds in a line directly behind another flap out of phase, avoiding the downwash of the bird in front.