Leaving for the fall
Seen from my neighborhood on a clear day, near dusk:
The birds were headed south, roughly parallel to the lake. Given the time of year, the size of the flock, and their great height (at least several hundred feet off the ground), I took it they might be leaving Chicago for the year. I don’t know what kind of birds I saw, but there is something about their situation–this was such a distinctive day in their lives, they were in for a dangerous trip filled with unknowns–that made me want to get just a few seconds on video.
In his comprehensive Bird Migration, Ian Newton writes about the advantages of the V-formation:
This is usual among geese, swans, gulls, cranes, pelicans, cormorants and others. Each individual flies behind and to the side of the one in front, benefiting from its slipstream, gaining lift and reduced drag. This is possible because each bird sheds vortices from its wing-tips which gives lift to the one behind. Individuals flying in V-formation have been estimated to save 12–20 per cent on energy costs compared with birds flying alone. The lead bird has no such advantage in power saving, and frequently relinquishes its position, pulling out and joining the line further back.
(Chapter 4, Migratory Flight)
Sources
Ian Newton, Bird Migration. Collins New Naturalist Library (Volume 113), 2010.