The migration of raptors has begun, and one of the first species to migrate in the fall is the Broad-winged Hawk (Buteo platypterus). While it is quite secretive when nesting, it is one of the more conspicuous species of birds when migrating. This is for two reasons. They are one of the few North American raptors that flocks during migration, and much of their migratory behavior is concentrated in the Northeast in a two-week period around the middle of September.
Migrating Broad-wings conserve energy by frequently soaring in thermals and mountain updrafts. Flocks of birds, or “kettles”, soar up the heated columns of air, peel off and glide to the next thermal where they repeat the process. Very little wing-flapping is necessary in order to cover a lot of ground. The flocks, or “kettles,” range from several individuals to thousands of birds (larger kettles generally occur nearer their Central and South America wintering grounds).
The number of birds migrating often grows following a cold front, when winds die down and thermals increase. Fall migration of Broad-wings in the Northeast is associated with good visibility, moderate favorable winds, high temperatures, and afternoons (vs. mornings). (Photo: juvenile Broad-winged Hawk)
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We are currently at the peak of the Broad-winged Hawk fall migration, an annual event that birders look forward to with great anticipation. These birds are gregarious, often migrating in flocks or groups called “kettles” that range from several individuals to many thousands of birds. In New England in mid-September it’s possible to see 10,000 Broad-winged Hawks a day at a hawk watch site; near the Great Lakes, 50,000 a day and Texas hawk-watchers have been known to see 300,000 to 500,000 a day.
Broad-wings depend more on thermals, rising columns of warm air, during their migration than most raptors. They don’t usually begin flying until mid-morning, by which time the sun has created thermals, and they stop flying as soon as thermal production ceases in later afternoon. During the day kettles can be seen circling around and around, higher and higher as they ride thermal columns of air upwards, peeling off at the apex and soaring (saving energy other migrating hawks use flapping their wings) southwards towards their wintering grounds.
To find hawk watch sites in your part of the world, go to the Hawk Migration Association (http://www.hmana.org/hawk-watch-sites/).
Naturally Curious is supported by donations. If you choose to contribute, you may go tohttp://www.naturallycuriouswithmaryholland.wordpress.com and click on the yellow “donate” button.