On a bright, warm afternoon in late September, a dozen or so bird watchers stood on the deck of the observation tower at the summit of Wachusett Mountain in Princeton and scanned the sky above them with binoculars and spotting scopes.
“They’re right above our heads!” one of them said excitedly.
As I looked overhead and squinted against the sun I saw what the birders were so excited about. Barely visible to the naked eye, about a thousand feet above the observation deck, a group of nine broad-winged hawks circled around each other in a vertical column known as a kettle.
After about five minutes, one of the birders announced that the kettle was breaking up, and the hawks drifted out of view.
This kettle was relatively small. In an interview a couple of days earlier, Wayne Petersen, director of Mass Audubon’s Massachusetts Important Bird Area Program, said dozens or even hundreds of broad-winged hawks can sometimes be seen swirling in a kettle, rising higher and higher into the air. And several kettles can pass through an area in a single day.
“A couple thousand broad-wings at Wachusett Mountain is the biggest number I’ve seen in a day,” Petersen said. “It’s kind of a crapshoot. Hawk watchers hope they’ll hit the mother lode and see several thousand broad wings in an hour. It’s a mystical and magical thing. If you hit it, you’ll never forget it.”
From late August to mid-November, thousands of hawks, which breed in areas north of Massachusetts, traverse our skies as they migrate south for the winter, bound for other parts of the United States, Mexico, Central and South America, and the Caribbean.
While broad-wings are the big show, other species commonly seen during the fall migrations include falcons, sharp-shinned hawks, American kestrels, ospreys, and northern harriers. Other birds of prey, such as turkey vultures and bald eagles, also join the migration.
Falcons, northern harriers, and sharp-shinned hawks typically use coastal routes when they migrate, Petersen explained.
“They’re power fliers. They head out over open water and island hop to Nantucket, Long Island, and so on. They’re not intimidated by water like buteos are.”
Buteos — a group of raptors that includes broad-winged hawks, red-tailed hawks, red-shouldered hawks, and rough-legged hawks — rely on columns of warm air called thermals that rise high above land, Petersen explained. Thermals provide lift that allows migrating hawks to soar.
“Buteos look for columns of warm air on sunny afternoons,” said Petersen. “Hawks sense where they are and move into thermals. And they look for other hawks riding thermals, so quickly dozens or hundreds of broad-wings can be seen swirling in a kettle, rising higher and higher into the air. It’s like an escalator to the hawks.”
When the thermal loses its energy, the soaring hawks drop in elevation as they power glide in a southwesterly direction until they find the next thermal to ride, Petersen explained.
“The hawks move from thermal to thermal, connecting the dots of warm air columns, picking up thermals at low elevation, then moving higher,” he said. “They can move huge distances in a single day if weather conditions are ideal.”
If the weather conditions aren’t conducive to thermals, such as on cloudy days, the migrating hawks look for ridgelines or isolated hills or mountains known as monadnocks, like Wachusett Mountain, which provide lift from wind coming off the slopes.
Many migrating hawks spend the night in forests quietly sitting in trees on mountains, said Petersen, and may be grounded for several days if the weather is not good.
Massachusetts’ premier hawk watching sites are Wachusett Mountain, Mt. Watatic in Ashburnham, and Mt. Tom near Holyoke, Petersen said.
Eastern Massachusetts Hawk Watch, a hawk conservation organization that monitors hawk migrations in Massachusetts, said on its website that Wachusett Mountain has averaged about 11,000 hawks a season since 1977. That average represents primarily the number of hawks that have been seen during the month of September.
“The peak for broad-winged hawks to migrate past most locations in Massachusetts and New England is roughly Sept. 12 – 22,” Shawn Carey, vice president of the hawk watch group, said in an e-mail. “By the time we hit October very few broad-winged hawks remain.”
Carey said that during October more sharp-shinned hawks, Cooper’s hawks, red-tailed hawks, turkey vultures, ospreys, northern harriers, bald eagles, and various species of falcons, such as peregrines, merlins, and kestrels, can be seen migrating.
A lot of hawks, including broad-wings, don’t tend to feed much during migration, said Petersen. Merlins, and accipiters, a group of hawks that includes Cooper’s hawks, sharp-shinned hawks, and northern goshawks, do feed, but not if they’re soaring. Merlins and accipiters feed mainly on other birds.
Hawks don’t have many natural predators, said Petersen, but great horned owls, and mammals like raccoons and fishers, can take hawk nestlings.
Threats to adult hawks include habitat destruction, electrocution from roosting on power lines, and being hit by cars when pursuing prey, like mice, beside roadways. Petersen said rodenticides that are used to kill rodents, can also kill hawks if the hawks eat rats or mice that have been poisoned.
We have 16 to 17 regularly occurring hawk species in Massachusetts, Petersen said. The red-tailed hawk, with a wingspan that can exceed 4 feet, is the largest hawk in Massachusetts. The American kestrel is the smallest, with a wingspan that can reach about 2 feet.
Petersen said in the past some hawk species fell victim to pesticides like DDT and some were even shot. But these birds of prey play an important role in the environment and are protected by the Migratory Bird Treaty Act.
“Most hawks feed on rodents and other small mammals, and some feed on insects too,” said Petersen. “They are beneficial as predators. These guys are not the enemy. They’re our buddies.”
Don Lyman is a biologist, freelance science journalist, and hospital pharmacist who lives north of Boston. Send your questions about nature and wildlife in the suburbs to firstname.lastname@example.org.