During this time of year, we get many questions about hawk identification. On October 2 at 11 am, Vince Smith will be presenting a Hawk ID class/field trip. In addition to identification, he will discuss optimal migration conditions and the variations in migration patterns in the various raptor species. You can view last year’s presentation from the link on our virtual presentations page.
In advance of this presentation, here is some information from Hawk Mountain and NJ Audubon so you can plan the best days and times to go to a hawk watch.
Migration costs raptors a lot of energy. Before migrating, some hawks gain as much as 10 to 20% of their body weight in fat as a high-density fuel for migration. Raptors conserve precious energy on migration by soaring—using rising currents and columns of air currents to gain lift and fly without flapping their wings. Raptors “slope soar” by riding winds deflected up and over hills and mountains. Birds “thermal soar” by circling in pockets of rising, warm air called thermals. Thermals are created when the sun differentially heats the Earth’s surface. Hawks ascend quickly to thousands of feet within thermals and then glide in the direction of their destination. Radar studies suggest that many raptors migrate at altitudes of 700 to 3,000 feet while soaring in thermals.
Large raptor “flights” are caused by the passage of a cold front—a high-pressure cell moving into the region from the north or west. The falling temperatures stimulate birds to migrate and the associated north to northwest winds ferry birds to the Atlantic Coast.
In general, the smaller raptors, like the American Kestrel and Sharp-shinned Hawk, are most common on the first day following the passage of a cold front. The larger, soaring birds, like the Eagles and Buteos, are more abundant on the second (or third) day of sustained north to northwest winds. The raptors that rely mostly on soaring do not start their migration until strong thermals begin to form after 8 a.m.
Northeast winds can also produce large flights, particularly falcons (which are not shy about crossing water and prone to migrate offshore). Southerly or southwesterly winds mean only several hundred birds a day during the peak of the migratory period, instead of several thousand birds.
Seasonal timing also varies by species. The peak passage of broad-winged hawks, which depend largely on thermal soaring, occurs in mid-September. Osprey, bald eagles, and kestrels are other early migrants. Large numbers of sharp-shinned hawks migrate in late September and early October. The peak of red-tailed hawk migration occurs in early November.
