National Geographic

Birds That Fly in a V Formation Use An Amazing Trick

Why do some birds fly in a V?

Most people would say that they do it to save energy, which would be right. But it turns out that birds in a V are actually pulling of a feat that’s more complicated and more impressive than anyone had imagined.

Here is the standard explanation for the V-formation:

As a bird flaps, a rotating vortex of air rolls off each of its wingtips. These vortices mean that the air immediately behind the bird gets constantly pushed downwards (downwash), and the air behind it and off to the sides gets pushed upwards (upwash). (See this image if that’s not clear.) If another bird flies in either of these upwash zones, it gets free lift. It can save energy by mooching off the air flow created by its flock-mate.

This all makes sense, but it represents decades of largely theoretical work. Scientists calculated how air should flow around a flying bird based on what we know about planes, but almost no one had taken any  actual measurements. Henri Weimerskirch changed that in 2001, when he fitted pelicans with heart-rate monitors. He found that birds at the back of the V had slower heart rates than those in the front, and flapped less often.

It was an interesting study, which confirmed that birds benefit from flying in a V. But it didn’t address why or how they do so. That’s what Steven Portugal wanted to know.

First, he needed the right technology. His colleagues at the Royal Veterinary College, UK developed tiny data-loggers that are light enough to be carried by a flying bird and sensitive enough to record its position, speed and heading, several times a second.

The devices had one problem: they don’t emit any information. If you strap them to, say, a flock of geese, the birds would fly off into the distance taking some very expensive equipment with them. “We needed a system where birds were actually migrating, rather than flying in a wind tunnel, but where we could get the data loggers back!” says Portugal.

Johannes Fritz had a solution. He works for an Austrian conservation organisation that is trying to save the northern bald ibis—a critically endangered species that makes vultures look handsome. The ibis went extinct in Central Europe in the 17th century, and Fritz is trying to reintroduce it into its old range. His team have reared several youngsters and teach them to fly along their old migration routes by leading the way in a microlight aircraft.

The human/ibis flock stops at fixed places along the route, and a support team follows them on the ground. That gave Portugal plenty of chances to fit the birds with loggers, record every flap of their wings for long stretches, and retrieve the data a few hours later.

The northern bald ibis. Credit: Ltshears.

The northern bald ibis. Credit: Ltshears.

The recordings revealed that the bird fly exactly where the theoretical simulations predicted: around a metre behind the bird in front, and another metre off to the side. Some ibises preferred to fly on the right of the V, or on the left. Some preferred the centre, and others the edges. But on the whole, the birds swapped around a lot and the flock had no constant leader.

But flying in a V isn’t just about staying in the right place. It’s also about flapping at the right time.

As each bird flaps its wings, the trail of upwash left by its wingtips also moves up and down. The birds behind can somehow sense this and adjust their own flapping to keep their own wings within this moving zone of free lift. “They trace the same path that the bird in front traced through the air,” explains Portugal.

Imagine that a flying ibis leaves a red trail with its left wingtip as it moves through the air. The right wingtip of the bird behind would travel through almost exactly the same path. “It’s like walking through the snow with your parents when you’re a kid,” says Portugal. “If you follow their footprints, they make your job easier because they’ve crunched the snow down.”

This is a far more active process than what Portugal had assumed. “We thought they’d be roughly in the right area and hit the good air maybe 20 percent of the time,” he says. “Actually they’re tracking the good air throughout their flap cycle. We didn’t think they could do that. It’s quite a feat.”

The ibises can also change their behaviour very quickly. As they switch places in the flock, they sometimes find themselves directly behind the bird in front, and caught in its downwash.If that happens, they change their flapping so that they’re doing the opposite of what the bird in front does. Rather than tracing the same path with its wingtips, it flies almost perfectly out of phase. “It’s almost like taking evasive action,” says Portugal. “They seem to be able to instantly respond to the wake that hits them.”

How do they manage? No one knows. The easiest answer is that they’re just watching the bird in front and beating their wings accordingly. They might be using their wing feathers to sense the air flow around them. Or they could just be relying on simple positive feedback. “They’re flying around, they hit a spot that feels good, and they think: Oh, hey, if I flap like this, it’s easier,” says Portugal.

Whatever the answer, it’s clear that this isn’t a skill the ibises are born with. When they first followed the microlight, they were all over the place. It took time for them to learn to fly in a V… and that adds one final surprise to the mix.

“It was always assumed that V-formation flight was learned from the adult birds,” says Portugal. “But these guys are all the same age and they learned to fly from a human in a microlight. They learned [V-formation flying] from each other. It’s almost self-taught.”

Reference: Portugal, Hubel, Fritz, Heese, Trobe, Voelk, Hailes, Wilson & Usherwood. 2013. Upwash exploitation and downwash avoidance by flap phasing in ibis formation flight. Nature http://dx.doi.org/10.1038/nature12939

More on Portugal’s work: Scientist Spills Water, Discovers Self-Cleaning Bird Egg

There are 18 Comments. Add Yours.

  1. Mary Arneson
    January 15, 2014

    If you’ve ever ridden a bike in a peloton, you know how being in the right spot just feels right. You don’t have to think about it, and if you move just an inch out of the “groove,” the extra work nudges you back into position. It’s got to be the same for the birds. Sure, you have to learn that there is a sweet spot, but once you get the feel for it, you don’t have to analyze anything to stay in it.

  2. Michael Day
    January 15, 2014

    I grew up in Virginia, and I remember asking my father why the geese flew in a “V”. He told me they only do that over Virginia and would switch to an “N” over North Carolina on their way south…

  3. Lori
    January 15, 2014

    Yes..this is all well and good…but do any of you know why sometimes one side of the V is longer?….
    [ pause to let a few guesses roll in..]

    Answer: There are birds on the longer side.

  4. Lori
    January 15, 2014

    oops.. i meant ‘There are MORE birds on the longer side’.

  5. JT
    January 16, 2014

    @Michael Day
    Great story, Mike. Great Dad.

  6. Leraillez
    January 16, 2014

    If we know why people walk in rythm without thinking about it, we will have a good idea for the birds flying.

  7. Slurpy
    January 16, 2014

    People walk in rhythm to attract the worm. . .

  8. Joe Duff
    January 16, 2014

    The dominance structure of birds in the wild and their ability to fly in formation is what maintains the flock. Stronger birds push their way to the front and consequently do most of the work. Less aggressive and weaker birds find the spot along the formation that fits their endurance all the way to the back where the weakest birds fly. By this method strong and weak birds can stay together. Otherwise they would be separated and all the birds in one flock would have to have the same flying abilities. When you lead the m with an aircraft this order is reversed. The man made wing created the strongest vertices so the bird closest to it gets the most lifassistamcet

  9. Joe Duff
    January 16, 2014

    I think the choice of flying on the right or left of the V is sometimes based on the wind direction. Flying in a cross wind from right to left means that the wake created on the right side is immediately pushed more behind the lead bird where it is disrupted by wake from its body and tail. Whereas the vortices from the left wing are pushed further from the bird and are therefore cleaner.

  10. Mary
    January 16, 2014

    Sometimes Canada Geese are in formation on the ground…pecking order?

  11. Mel Healy
    January 17, 2014

    I second that observation about the aerodynamics of the peloton. Watch the way it “flocks”, splits and regroups when it encounters a roundabout, or how it angles itself in a crosswind.

  12. Dhirendra Devarshi
    January 17, 2014

    I can agree on this theory grossly but has any one tried to find an answer to the fork (or V) not getting split further, I mean if it is all due to aerodynamics one would expect occasional branching once the arms have grown long enough. In a long arm of ‘V’ an individual somewhere in the middle of arm can give rise to another ‘V’ just like the one (or first) heading the ‘main V’! Will this create some kind of aerodynamic hurdle or not found suitable because it blocks the view? If so what about the birds who fly in compact flocks like Mynas? We need to explore more about group flights. Do we have answers to all these questions?

  13. Reza
    January 17, 2014

    This must be the same thing dolphins are doing when they swim along the side of boats. No, they don’t love you– they’re just using you.

  14. Nigel Calhoun
    January 17, 2014

    I think the birds are flapping in sync with the upwash which is cycling up and down with flapping rhythm of the bird ahead. This provides lift at the top of the flap as well as the bottom – greatly reducing the energy expended. The same upwash cycle is passed backward to the next bird. The birds can feel the upwash and flap with it rather than against it.

  15. Dave C
    January 18, 2014

    Often this kind of research just seems to fly off the silly end. You don’t need much more than a bird brain to see that to a bird the firmer air in the right spot just feels better, the same way we would just naturally walk on the firmer part of a path and not the mushy parts.

  16. Dave C
    January 18, 2014

    So do the same birds always fly on the same side of the v ?
    It seems to me that a wise bird would switch sides occasionally to prevent developing a weak wing on the inside, since the inside wing has a lighter work load. I wonder if they gathered any data on that.

    [From the post: "But on the whole, the birds swapped around a lot and the flock had no constant leader." - Ed]

  17. Gerri Ford
    January 19, 2014

    I guess I am the only person who was told that birds fly in the V-formation so they could all see the leader in the front of the flock. Somehow I always knew there was a more complicated reason.

  18. Paulino José Soares
    January 19, 2014

    The correct common name for the species is Northern Bald Ibis.

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