Dec 23, 2019
Essential Equipment for Migration
“Thermals” are not just another name for long underwear. They’re also what make landing at airports such a bumpy affair. And whereas pilots have to compensate for thermals in their approach, birds often count on them for their migratory departures.
Even small differences in the landscape create various heating patterns in the air. A plowed field absorbs heat more quickly and releases it more slowly than a planted field or forest. Bodies of water, large and small, respond differently to the sun’s effects. Small towns and housing developments have a different heat index than large concrete jungles. And over some parts of the landscape, a kind of inverted funnel of fast-rising air bubbles up to elevations from 1,000 to 3,000 feet, creating a natural elevator to the skies above.
Many birds- especially large, soaring daylight migrants – take full advantage of these thermals. Through experience, instinct or luck, a bird will find one of these updrafts and take it upward to its peak. From there the bird soars, losing altitude slowly as gravity takes over, until it finds another thermal and again rises through the turbulent air. This very energy-efficient way to travel makes it possible for some birds to migrate long distances without a large fat reserve.
At any time of year, you may see a single hawk, or even a pair, lazily floating on thermals as they travel about looking for food. But during migration, you can see dozens, sometimes even hundreds – of birds using the same thermals in their efforts to push ever southward. This can be particularly impressive when Broad-winged Hawks are migrating en masse. Groups of migrating birds climbing the same invisible ladder are called “kettles”, and kettles are one of the most visible signs of migration in process. Watch for them – hawks, vultures and falcons circling higher and higher in the sky, seemingly static, but actually headed south to Louisiana to Mexico, to Peru and Brazil and other wintering havens, using nature’s elevator to speed them on their way.