I was reminded by a flock of Sandhill Cranes flying overhead that it’s that time of year again – migration. As the days lengthen as winter wanes, birds in northern climes get restless and finally uproot themselves from their southerly wintering areas. It’s always a sight to ducks and geese and cranes flying in their V or semi-V patterns. But why those patterns?
Many years ago when I was a kid watching nature or hunting shows popular at that time, the TV screen would show a V skein of geese and the announcer would say something like “and if the leader of the flock is killed, the flock would be lost.” The idea was that the first bird was the leader and, being more experienced, knew the way to the wintering grounds. Or that the first bird “broke the trail” and made flying easier for the rest of the flock. Actually, migrating birds have no leader; watch and you will see that the lead bird changes often. All birds in a flock flap away against the friction of the air, producing lift but also drag. The advantage to formation flying lies in what pilots call the “wingtip vortex.”
On a downstroke, the wing of the bird pushes air downward; physics requires that the same volume of air be displaced upward. These movements produce vortices (little whirlwinds) at the ends of the wings and birds that are adjacent to and a bit above the bird in front receive the benefit of uplift, reducing their energy use by 30% or so. See more information at Aerospace Web on why birds fly in a V.
Songbirds and shorebirds don’t fly in organized flocks. Large flocks of crows, blackbirds, starlings, and sandpipers fly around during the day and birds like warblers, thrushes, and orioles migrate at night. They move together but not in any particular formation; their wings are so small compared to their body size that the aerodynamic advantage that geese have does not apply.
How high do migrating birds fly? Well, it varies. Smaller birds have to work harder because they have a smaller wing to body ratio, so they tend not to fly as high. Winds change direction with altitude, so birds will pick the altitude that most favors them, but the higher a bird goes, the thinner the air is and the harder it is to fly. Bird that have to traverse a mountain range obviously have to go higher, but none higher than the Bar-headed Goose that crosses the Himalayas at 29,000 feet!
How far does a bird fly on a leg of migration? Maybe only a few miles but none farther than the Bar-tailed Godwit that breeds in Alaska and flies 6000 miles nonstop enroute to its wintering ground in New Zealand! One female godwit took nine days, non stop, to reach its destination.
Bird flight speeds on migration vary from about 20 to 40 mph. Canada Geese fly at about 30-40 miles per hour while migrating; smaller birds fly at under 30 mph.
For a more formal and scientific discussion of flight, see this researchgate article.