Why do geese fly in a "V" formation?

Canada goose, one of many flying in a familiar "V" formation.

Nature has its secrets, and though many are familiar to us, we rarely understand why something behaves the way it does.  Many of those secrets have to do with unseen physics.  The way clouds form, the way eddies form in in a stream, or the way that geese fly in a V-shaped pattern.  Wait.  What does the shape of a flock of airborne geese have to do with physics?

It all has to do with the turbulence and air pressure changes that occur as a goose beats its wings.  There is an excellent video here, but I can tell you the short version.  

When the bird raises its wings it causes the air trailing it to form a pressure wave called an upwash.  When the bird lowers it wings the pressure wave forms a downwash.  The waves occur behind the bird and are angled away from it; there is no such turbulence directly behind it (streamlining).  A bird following the lead will take advantage of the upwash by riding in its wake and gains a bit of lift in the process.  It often flies just slightly below the level of the first bird.

Following birds will continue the pattern, each in turn taking advantage of the upwash from the previous flyer.  The problem comes with the extra effort the lead bird must employ; it will tire faster than the others.  To resolve that, the birds take turns leading.

I took the above shot yesterday.  Below is another shot, this of a group of geese flying at right angles to me.  The "V" pattern is not easy to see, but it is there.

Thanks for reading.

Eric Svendsen     www.ericspix.com