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