Birders track raptor migration across Detroit River

Each fall a river of hawks pours across the mouth of the Detroit River.

Thousands of raptors like the American Kestrel, Bald Eagle and Turkey Vulture, cross the river every day.

And each one is counted by hand.

The Detroit River Hawk Watch tackles the job. It’s currently a collaboration of the U.S. Fish and Wildlife Service, the Detroit River International Wildlife Refuge and the International Wildlife Refuge Alliance. The group’s goal is to count 16 species from two locations as they cross the river’s mouth from September 1^st to November 30th.

The count started in 1983 with just one man, but now includes professional counters, volunteers and international recognition for the high volume of birds that pass through.

“It is well established in North America, actually you know, it’s internationally known as one of the largest passages of broad-winged hawks,” said Greg Norwood, a biological technician of the Detroit River International Wildlife Refuge, who leads the watch.

Broad-winged hawks made up about 71 percent of raptors counted from 1991 to 2008 — more than 2.5 million of them.

Last year one professional counter and 16 volunteers took turns scanning the sky 10 hours a day identifying and counting birds.

And that’s not an easy task.

“You’re looking at as many as, you know, 10,000 specks in the air. And you’re basically trying to systematically count those,” Norwood said.

In the late 90s about 500,000 birds funneled through in one day.

“It really is impressive to watch and very memorable especially in November when you might see a golden eagle pass over head and we see so few of them in Michigan,” said Jerry Jourdan an avid bird watcher and photographer. He also serves on the watch’s advisory committee.

To the raptors, crossing the Detroit River is just another four miles on their long winter migration south from Canadian provinces like Ontario, Quebec and Manitoba.

To save energy the birds glide between “thermals” or rising pockets of warm air.

Since these pockets don’t form over water the birds avoid flying over the Great Lakes. Instead they travel east of Lake Huron across Ontario. But when they reach the Detroit River the birds are forced to cross it.

“They’re absolutely losing all their thermal activity and because it’s four miles of open water, and thermals don’t form over water, they hit our site as they’re trying quickly to find another,” Norwood said.

Without this uplift of air the birds descend, making counting them easier.

“So they’re streaming out in these lines and you can count them in hundreds or tens as they glide over your head,” Norwood said, “That’s kind of what we see.”

The data collected aids in monitoring population trends – like that of the turkey vulture.

“They are a species that we’re interested and in fact they’re the ones that we’re able to show that they’re statistically increasing in abundance,” Norwood said.

In 2008 nearly 70,000 turkey vultures were counted compared to about 37,000 counted in 2001. The count first employed a full-time counter in 2001.

“There’s something going on with them, they’re responding very well to some kind of landscape change,” Norwood said.

The hawk watch hopes that local population trends and data can help identify larger, continental trends.

It’s about “…mastering our own information so that we can contribute to a bigger picture,” Norwood said.

Like contributing to the Raptor Population Index, a raptor population database made up of hawk count data across North America.

“The raptor population index is really designed to give us a sense of how robust populations are,” said Gil Randell, board chair of the Hawk Migration Association of North America, an organization that helps manage the index.

As top predators, raptors make good environmental gauges for factors like the accumulation of toxins.

“All sorts of environmental impacts in general, they are sensitive to that,” Norwood said.

Others agree that this is important. But they count hawks for other reasons.

“I’m not involved because of my love for mankind and my love for the planet, although I do love mankind and the planet,” Randell said. “I’m involved because I just think that the resource in and of itself is so important so exciting and so rewarding to study.”

Hawks migrate through other Great Lakes states and provinces. Find out where on this map.