How do migrating birds travel afar with barely a flap of the wings?
A. One way is by free-riding on thermals, columns of heated air uprising when the sun warms the ground unevenly, says Chris McGowan in “Diatoms to Dinosaurs: The Size and Scale of Living Things.” A bird could take one of these “elevators” high, then glide down many miles until finding another thermal, and back up to glide some more, etc. Thermals work best over land during the heat of the day. Early morning thermals are weakest, when light birds with low wing-loading take to the skies. Later, larger and larger soarers can go; then at day’s end, landings are reversed, until last grounded are the lightest birds as the thermal system peters out. Locating thermals is not always easy, so birds will watch for other birds rising and circling. There can be surprises, however, says the Centennial Museum of the University of Texas, El Paso, as when one flock watched a very big flier stay aloft, circling with ease. “How frustrating to watch a helicopter obviously using a thermal that you just can
