ACADIA NATIONAL PARK — Every weekday from mid-August to late October, people who are fascinated by birds of prey take their binoculars and perch on an east-facing ledge near the summit of Cadillac Mountain.

In just a couple of hours one morning last week, they spotted 185 birds: bald eagles, osprey, American kestrels, broad-winged hawks and a steady stream of sharp-shinned hawks migrating south.

This is the 25th year of Acadia’s Hawk Watch program. It is both for the education and enjoyment of park visitors and the collection of data to inform research into bird migration patterns and population trends.

“Raptors are good indicators of what’s happening in the environment,” said Seth Benz, who was involved in starting the Hawk Watch program and is now the bird ecology director at Schoodic Institute.

Since 2013, the program has been a joint effort of the institute and Acadia’s interpretive division. A park volunteer is at the Hawk Watch site every day to talk with visitors and make sure each sighting of a raptor is recorded. The volunteers are organized by Carol Thompson, a volunteer herself.

The east side of Cadillac Mountain is ideal for hawk watching because it is in the migration path of a number of raptors and the wind often gives them a lift. Broad-winged and sharp-shinned hawks, especially, use the updrafts created when wind strikes the side of a mountain such as Dorr and Cadillac.

“The birds can save energy by soaring with the updrafts and use that to move southward without a whole lot of flapping,” Benz said.

Broad-winged hawks also use thermals, pockets of air that rise from warm places on the ground, such as rock outcroppings, to gain altitude.

“The birds get into these thermals and rise to the apex,” Benz said. “Then they glide out of that and go toward the next thermal they can see, which may be miles away. Birds of prey have really good eyesight, so they often find a thermal by seeing it pick up things like leaves or insects.”

The broad-winged hawks do that all the way down the Appalachian Mountain chain and through Texas and Mexico to South America.

Benz said scientists are interested in whether the timing of raptor migration is being influenced by climate change.

“We’re closely watching for when these birds come through. Is that changing at all because of a warming of the earth? Is that shifting the thermal development in the season, and do the birds indicate that is occurring?

“This is one of the nuances, one of the reasons we should be watching for hawks,” he said.

Benz said sharp-shinned hawks don’t use thermals as much as their broad-winged cousins.

“They generally use more of the updraft migration mode. They have the flap, flap, flap, and glide kind of style.”

At most designated raptor-watching sites in North America, participants are constantly looking up at what Benz describes as “little pepper specs in the sky.”

Hawk watchers do that at the Cadillac Mountain site, too, but they also look down into the gorge between Cadillac and Dorr mountains. Birds that rise up over Dorr often glide down into the gorge before riding an updraft over Cadillac.

“Being able to look down on these migratory raptors is a thrill for a lot of the visitors who come by and why some people come back year after year,” Benz said.

“There are people who have been coming for 20 years or more. They take a vacation to come this time a year and spend a week or 10 days watching hawks. That’s dedication.”

The Acadia Hawk Watch program follows the data collection protocols of the Hawk Migration Association of North America. Anyone can keep up with the daily raptor counts from Cadillac Mountain at hawkcount.org.

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