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