One of the fascinating things about bird migration is the patterns in which the birds fly and maneuver in a group as they move along. We are all familiar with the classic V-formation of bird flight, which we have been told is the most aerodynamic way for large groups of birds to fly together. But how exactly does the V-shape work, and how are the birds using it?
I happened to be sorting through a bunch of images of large birds flying in groups together and noticed that there didn’t seem to be a consistent pattern in their wing movements from bird to bird. In fact, it looked disorganized rather than the synchrony I had expected.
In a unique study of imprinted young Bald Ibis that were being trained to fly from their breeding area in Austria to a wintering area in Italy, transmitters were fitted on the birds to provide data on their flight mechanics during V-formation flying.*
Bottom line: what matters is how close and where (left, right, or center) a bird is relative to the bird in front of it. As the lead bird flaps down, it pushes that air up and over its wingtips; the bird behind can take advantage of that updraft (as lift) if it positions itself a certain distance behind and just to the right or left of the lead bird. Therefore, it doesn’t need to flap downward as hard in order to stay aloft. And that is the energetic savings of following rather than leading. How simple! But proximity behind the lead bird is critical, because the updraft from the wing tips is spatially limited.
If you think of the pressure wave of the downward wing flap of the lead bird as a sine curve, the best lift is achieved if the following bird stays in the same place as the lead bird on that curve. It’s similar to the “push” you get by drafting at an angle off a bicyclist just ahead of you. Since the lead bird is continually flapping, the following bird must continue to flap in exactly the same phase in order to get the benefit.
However, if following birds are too close or too far from the lead bird or directly behind the lead bird, synchrony is actually less efficient because instead of catching the upwash air, it might be catching the downwash instead — which would necessitate flapping harder and expending more energy.
A fascinating summary of this unique study on the Bald Ibis appeared in Nature News, with a video that more clearly explains what I have tried to describe above.
*S. J. Portugal, et al. Upwash exploitation and downwash avoidance by flap phasing in ibis formation flight. Nature, 16 Jan 2014.