Soundtrack by the Grateful Dead (1974)
Big flocks of Sandhill Cranes are flying over, heading south for the winter. Apparently there's a thermal rising here because they are coming in at a lower elevation, catching the thermal, and riding it up in a circle until they are several hundred feet higher. Then they head south, all the while singing their beautiful song.
I have yet to determine how the date of their migrations north and south correlates with the severity of the winter.