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 off 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 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