Geese fly in V formations.
This lets each goose (except the lead, of course) surf on the wingtip vortex generated by the goose ahead, reducing the amount of energy needed to stay aloft. The aerodynamics of wingtip vortices is well-understood. It's the reason that most modern airliners have "winglets" - those small vertical blades at the ends of their wings.
The winglets reduce the size of the vortex produced by the wingtip, and therefore reduce the drag.
Of course, geese don't have winglets - they're gonna create vortices. But as a group, they can work together. Like NASCAR drivers "drafting", geese maintain a precise relative position to take maximum aerodynamic advantage.
But did you ever notice that the V is almost always asymmetrical? One side is usually longer than the other. Turns out there's a precise mathematical reason for that, too.
There's more geese on that side.