The In-flight Answer

Dear Matthew:

On my way to work the other day, I was watching what I think were seagulls flying in a V formation. I've seen ducks and geese do this, but it suddenly occurred to me that I've never seen other birds like sparrows or pigeons fly that way, even though they hang around in flocks like geese and ducks. Is there a reason for this? Can you find out what it is?

BG, Vista

The geese stonewalled us, but ornithologists and aerodynamicists have studied formation flying, of course, and they're only too happy to go on at length for the benefit of us lesser mortals. As usual, the answer you get depends on who you ask, truth being a slippery commodity when it comes to the whys behind animal behavior. I'll go with the generally accepted notions.

On page one of the flight instruction manual it says, the more you weigh, the harder you have to flap. The average hanging-around bird weighing only a few ounces needs to maintain an air speed of 11 to 15 miles per hour to keep itself airborne. If an ostrich could fly, sez one scientist, it would have to zoom along at 100 miles per hour to keep all 250 pounds off the ground. By extension, the most aerodynamically well-designed scientist would probably have to maintain 65 or 70. At any rate, flight is the most demanding physical stunt a bird can do. Any edge it can get it will take. Formation flying is one such advantage.

In an effort to find out what that advantage is, the lab coats devised a clever little calculation called Munk's Stagger Theorem. This allows a scientist sitting on his butt in front of a computer to tell geese how to get the most out of flight. The fact that geese don't fly in strict compliance with theory just means they still know something we don�t. As it should be, I think.

Like planes, birds create turbulence behind them as they fly. Air coming off the top of a bird's wing produces down-turning eddies of air. Any companions flying too close on a goose's heels would have to work even harder to stay at the proper altitude and keep up with the flock. But the tip of each bird's wing creates vortexes that turn in the opposite direction, creating an updraft. A bird flying above and just off the end of its neighbor's wingtip benefits from the buoyancy. Taking advantage of this, a flock of two dozen geese can increase its range by as much as 71 percent.

Another advantage of V-formation flight is its self-correcting quality. Any bird that surges too far ahead of its position is slowed down by the lack of lift and naturally falls back into place.

Why geese and not sparrows? Most common sparrows don't fly any farther than the next outdoor restaurant, so they don't have much reason to worry about long-distance aerodynamics. And as handy as the wingtip vortexes are, they disintegrate fairly quickly. The larger the wing, the longer the effect lasts. For sparrows to take full advantage of the updrafts, they would have to fly too close together, turning the well-ordered flock into a 30-car pileup