To save energy? Well, yes, there's that. But the energy-conservation hypothesis went largely untested for many years. Now, results from a rather creatively designed experiment have confirmed what we've long supposed about flying formations – but not without raising some downright baffling questions in the process.

Ed Yong dissects some of the finer points of the study, which was conducted by researcher Steven Portugal and his colleagues at the UK's Royal Veterinary College by attaching sensors to endangered ibises and teaching them to migrate with the help of a microlight aircraft:

[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."

Add to this the fact that the birds learned to assume this formation not from an older bird, but from a human-operated microlight, and you have yourself a more confusing puzzle than the one you started out with.

Read the rest of the details over at Nature and National Geographic.