Synchrony

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.

A flock of Canada Geese flew toward me in a quasi-V-formation in August, 2020. Young birds often “train” with their parents before actual migration begins, both to get their flight muscles in shape, and to learn how to fly efficiently in V-formation.
A different flock of Geese looked a little better organized in their V, but the wing flaps of individual birds didn’t match up.

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.*

Bald Ibis chicks were imprinted on humans and trained to fly behind an ultra-light aircraft. Data from GPS transmitters attached to the backs of the birds was used to assess whether birds truly were making use of the V-formation for aerodynamic reasons. Photo from study by Portugal et al. 2014 in Nature.

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.

Perfect wing synchrony in these Trumpeter Swans as they fly in a tight formation.

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.

Wing beats of lead and following birds are synchronous (in the same phase) when the following bird catches the “upwash” of air from the lead bird’s wing flap.

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.

Wingbeats are asynchronous and opposite (out of phase) when following birds are too close to the lead bird or flying directly behind them. In this case, the following bird positions itself to stay out of the downwash of the lead bird.
Flying close together makes more sense, as long as it’s not too close!

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.

4 thoughts on “Synchrony

  1. Thanks for this post! I vaguely recalled that there was something aerodynamic going on, from an old NOVA show, I think, but could not recall exactly what it was! Good to see some more on it.

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