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.”
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 birds 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