By Jay Ingram
Aerodynamic efficiency, it turns out. After years of speculation, scientists prove that birds fly in V formation to increase their range
By now, depending where you live, you will have seen Vs of birds returning to the Great White North, especially Canada geese. Why do they fly in Vs? A number of my friends have voiced the opinion that, of course, it’s for aerodynamic reasons, but until now their confidence has been unfounded. Yes, it has been speculated that flying in Vs is an energy-saving move, but solid evidence has been extremely hard to come by. However, a brilliant new study from the Royal Veterinary College has pretty much nailed it: birds do fly in Vs to get a lift.
The trick is for the big guys
With the advantages larger birds gain from flying in V formations, why don't we see the habit in smaller birds? Once more, aerodynamic analysis provides the answer: the size of the upwash behind a wing — the key factor in the efficiency of formations — depends on the size of the wing itself. The smaller the bird, the smaller the wake. The rest is simply diminishing returns.
Why was it so difficult to establish until now? More on that shortly, but let’s take a detour through the general theory first.
Intuitively, it makes sense that birds like geese would fly in Vs.Vortices of air spilling off the tips of one bird could lend lift to the wings of the bird behind. The math even predicted it. A branch of mathematics developed to analyze the aerodynamics of aircraft was applied to geese, and the calculations showed that a flock of 25 birds, flying tip to tip, could increase their range by 71 per cent over individual birds. A group of nine could gain 50 per cent and even three would get a benefit.
The reason? Each goose creates turbulence, and eddies swirl away from the tips of its wings as it flies. The air immediately behind the wings is moving down (downwash), but at the sides, just beyond the wingtip it’s moving up — and rapidly, too.
However, the intensity of this upwash falls off pretty quickly. A bird more than a metre away isn’t going to derive much benefit. Nor are the benefits shared equally. A mathematical expression called Munk’s stagger theorem showed that, in a V, some birds win and some lose. Lead birds lose the advantage of having birds in front of them, for instance, while birds further back gain an advantage even from those two positions ahead.
The shape of the formation counts, too. Theory says that the best V would actually be a little bent, with the tip birds drifting slightly back and out from the main line, and the lead bird slightly back from the apex. The angle of the V should be roughly 100 degrees, about the angle your little finger makes with your thumb if you stretch your hand as wide as possible, but it does not have to be symmetrical. A V with 10 birds in one arm and 15 in the other would require only slight adjustments in spacing to work.
The really neat thing is that it’s easier to maintain the V shape than it is to disrupt it. If a goose flies too far forward out of the V it will lose uplift and fall back into line. If it drifts too far back, it will get more lift and suddenly find itself surging forward back into its proper place.
But all of this is just the theory. Even though fighter jets apparently can save as much as 18 per cent in fuel consumption by flying just behind the wingtips of another, they aren’t geese. You can’t put geese in a wind tunnel to study their aerodynamics. And photographing them outdoors is always problematic, because it’s tricky to measure the angle of a V — it depends on exactly how the V is oriented with respect to you.
So the benefits of V formations remained unproven until the Royal Veterinary College scientists outfitted 14 northern bald ibises in Europe with custom data trackers that recorded every single wing flap and every shift of body position for 43 minutes straight. The results, published in January in Nature, were unequivocal: the ibises kept their bodies in the ideal aerodynamic position and timed each wingbeat so as to take maximum advantage of the air currents spilling off the wings of the birds around them. Birds in the V kept their inner wing in the upwash zone of the birds in front, just as theory had predicted.
Even better evidence is the fact that when birds change places on the V, they sometimes move exactly behind the bird in front, placing them in the unhelpful downwash. When that happens, they switch their flapping pattern to be out of phase with the bird in front and so minimize the negative effects.
So it looks like these ibises, and presumably all other V flyers, do use the formation to gain an advantage. But how important is it? My knowing friends will likely say, “I told you so.” But there’s still more to the story. For instance, other research has shown that pelicans’ heart rates slow when they’re flying in formation, and such research reveals something just a bit more intriguing at every step. So let’s hear more!