Why Geese Fly in a V

[A] A wing does not push air straight down. It leaves behind a pair of rotating trails, one from each tip, and just outside each trail the air is moving upward. [B] A bird positioned in that rising air gets part of its lift for nothing. The V is the shape that places every bird except the leader in such a position.

The saving is substantial. Measurements on pelicans flying in formation recorded heart rates noticeably lower than those of birds flying alone at the same speed. [C] It also explains the otherwise puzzling detail that birds in a V flap in a coordinated rhythm rather than independently: the upwash they are exploiting rises and falls with the beat of the wing in front, and a bird gains most by timing its own stroke to it. [D] The leader gets no benefit, which is why the position rotates during a long flight, though the mechanism by which a flock decides on the change is still not well understood. Radar studies suggest that the change is initiated by the leader rather than demanded by the birds behind.