September 18, 2015
Many raptors, like golden eagles, migrate solo. But broad-winged hawks travel in groups. When the flock is large, it takes on the appearance of a “kettle"—a large spiral of circling birds that swirls like liquid in a pot that’s being stirred.
Why do they do this?
Raptors save energy by gliding. To reach the right altitude, they find a column of hot, rising air and hop on the thermal updraft. Within the thermal, new arrivals enter low and rise like bubbles in a boiling pot. Going up, they soar tightly to stay inside the thermal. At the top, where the air stops rising, they start their glide. At the end of the glide, they find another thermal and start again.
Right now, millions of broad-winged hawks are migrating in a very concentrated two-week timeframe. Many are moving along the shores of the Great Lakes. They avoid flying over large bodies of water because thermals only set up over land.
As they travel south and the continent narrows, the flocks grow. By the time they reach Veracruz, Mexico, more than a half million birds can pass through the region each day.
But even with that many birds, you won’t hear them. Their migration is nearly silent. On the breeding grounds, however, you would hear their call—a high-pitched whistle.
They’re moving through Pennsylvania in large numbers this week. The best chance to see them is at Hawk Mountain, west of Allentown. Closer to Pittsburgh, check out the Allegheny Front Hawk Watch near Central City.
Bird songs and calls for the audio recording of this segment are provided by the Macaulay Library at the Cornell Lab of Ornithology, and recorded by Geoffrey A Keller.