Capital News Service
LANSING — Each fall a river of raptors pours above the mouth of the Detroit River.
Thousands of birds like the American kestrel, bald eagle and turkey vulture, cross the river daily along a migration route that has an international reputation for the high volume of birds that pass through.
And each one is counted individually.
The Detroit River Hawk Watch tackles the job.
It’s a collaboration of the U.S. Fish and Wildlife Service, Detroit River International Wildlife Refuge and International Wildlife Refuge Alliance. The goal is to count 16 species from two locations between Sept. 1 and Nov. 30 to help scientists track local and continental population trends.
“Actually, it’s internationally known as one of the largest passages of broad-winged hawks,” said Greg Norwood, a biological technician at the wildlife refuge, who leads the watch.
Broad-winged hawks made up about 71 percent of raptors spotted 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 to identify and tally birds.
That’s a tough task.
“You’re looking at as many as 10,000 specks in the air. And you’re basically trying to systematically count those,” Norwood said.
Sometimes there are a lot more specks. For example, one day in the late 1990s 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 overhead, and we see so few of them in Michigan,” said Jerry Jourdan, an avid bird watcher and photographer from Wyandotte who serves on the watch’s advisory committee.
For the raptors, crossing the four-mile mouth of the Detroit River is just a small leg of their long migration south from Canadian provinces like Ontario, Quebec and Manitoba to places like South America and Mexico
To save energy, the birds glide between “thermals,” or rising pockets of warm air.
Since those 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.
Norwood, of the wildlife refuge, said, “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.”
Without that uplift of air, the birds descend, making it easier to count them.
“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.
The data collected aids in monitoring species like that the turkey vulture.
“They’re a species that we’re interested and, in fact, they’re the ones that we’re able to show statistically increasing in abundance,” Norwood said.
In 2008, watchers counted nearly 70,000 turkey vultures, up from about 37,000 in 2001.
“There’s something going on with them. They’re responding very well to some kind of landscape change,” Norwood said.
The hawk watchers hope 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.
That includes contributing to the Raptor Population Index, a database of hawk count data across North America.
The index is designed “to give us a sense of how robust populations are,” said Gil Randell, board chair of the Hawk Migration Association of North America.
Norwood said that, as top predators, raptors make good environmental gauges for factors like the accumulation of toxins.
Others agree that’s important, but say they count raptors for other reasons.
For example, 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.”
Alice Rossignol writes for Great Lakes Echo