Why Birds Fly in Vees
Any day now, the geese, the ducks, the cranes and other transient summer guests will be leaving us to spend the winter in a more sensible climate. One cannot help but feel a little wistful as he watches those majestic vees march across the sky amid the honking and quacking and screeching, and the question always comes to mind: Why do they fly in formation?
When I was a boy, I was told that Papa Duck was always at the head of the vee with the rest of the family spread out behind. Actually, that idea is not much more fanciful than some others that sound a lot more scientific. The favorite incorrect theory is that the lead duck is "breaking trail" for those that follow, in much the same manner that racing cars "draft" in the wind shadow of the leading car. Then, so the story goes, when the leader tires, it drops back in line and another duck takes its place at the head of the vee. That isn't the way it's done.
In actuality, when migrating birds fly in a staggered pattern, each is subjected to the same amount of air friction as its neighbor. The advantage to formation flying lies in what pilots call the "wingtip vortex."
On a downstroke, the air beneath a wing is displaced downward, but in order for the volume of air surrounding the bird to remain the same, some must also be displaced upward. This creates an upwash beyond the wingtip which results in a favorable effect for the bird's neighbor. Each flies in the upwash of neighbors--an effect similar to flying in an upcurrent, with less lifting power needed.
If this is so, it might be asked: Why don't they just fly line abreast, instead of in a vee? The answer is that the line abreast formation does not result in an equal saving of energy for each bird. A study published in Science magazine shows that, for line abreast formations of ten birds or more, those in the center of the formation enjoy about twice the advantage of those at the tips. This is because the center birds are flying in an upwash field generated by neighbors on both sides.
The vee formation balances things out. In a proper vee, each bird is expending the same amount of energy. This even holds for the lead bird, which intuition would tell us receives no help whatsoever from its neighbors because they are behind it. It does though, but if it should happen to move ahead out of the upwash field, it merely drops back until it is in it again.
This applies to any bird in the entire formation. If a member should move ahead of the vee line, it finds that more power is required to keep up with the flight and its speed falls until it is back in line. Thus, the vee is self-stabilizing, and even young birds who have just begun to solo will immediately adjust to any slot in the formation so that it "feels right."
Vees need not necessarily be symmetrical, because only the influence of the neighboring half-dozen birds is significant. The important thing is that the front bird should have others on both sides. In other words, one must always have a vee apex, but the legs can be different lengths. The clever bird, if it understood all this, might think, "Aha! This tells me that, if I flew inside the vee, I would get the benefit of all those dumb clucks around me." Apparently, though, flights do not tolerate malingerers who attempt to use less power at the expense of more work for the main formation. It is possible that exceptions may be made for sick or weaker birds.
The savings in energy brought about by formation flying can be truly significant. The study in Science reports that a flight of 25 birds can achieve a 70 percent range increase over a bird flying solo using the same amount of energy.
At the same time, it is interesting that formation flights cruise at an economical speed 24 percent less than the single bird who is, possibly, frantically trying to catch up with the main formation.