Bald eagles are continuing their treetop comeback in the Twin Cities stretch of the Mississippi River, where an aerial count this week found nine more nests than a year ago, with at least 48 nesting pairs.

The number has risen steadily from 21 when the National Park Service began aerial counts in 2007, the year the bald eagle was removed from the endangered species list after 40 years. Based on historic surveys and growth rates, Minnesota has an estimated 6,000 bald eagle pairs, said Lori Naumann, a nongame specialist with the state Department of Natural Resources.

"We continue to marvel at how much the population has increased," said Bill Route, a Park Service ecologist who counted nests Tuesday.

An intriguing sign of the eagle's growing urbaneness is a new nest built in the heart of downtown Minneapolis, just upstream of the Interstate 35W bridge over the Mississippi.

"I was just amazed," Route said. He spotted the nest Tuesday from a two-seater plane piloted by a park ranger. They banked above the river gorge near the Lower St. Anthony Falls with "20-floor skyscrapers on one side and a big radio tower on the other and there's that nest right there."

The continued rise in the birds' numbers "reflects the improving quality of the Mississippi River," as well as a growing tolerance by people and eagles for each other, said Mark Martell, bird conservation director for Minnesota Audubon.

"We are getting very urban birds," Martell said. "I'm guessing this will continue as hatched birds grow up in an urban area. … It will seem normal for them."

The Park Service count covers 72 miles from Dayton to Hastings, a stretch known as the Mississippi National River and Recreation Area.

Route and park ranger-pilot Steve Mazur made the four-hour count Tuesday. After the official count, during a 15-minute flight with a reporter aboard, an immature eagle flew across the river toward Inver Grove Heights. Eagles don't breed until about age five and immature eagles aren't reflected in the nest count. The raptors mate for life and usually return to the same nest each spring.

Bald eagle sightings were common this winter in downtown Anoka, where one was perched in a tree not far from the Main Street Bridge over the Rum River, resident Jeff Weaver said.

Weaver said he recently saw a stunned mallard crash onto Hwy. 169 near the Main Street bridge, followed by an eagle that grabbed it and flew to a nearby oak for a duck lunch.

"The winter was a phenomenal display of the energy and prowess of these eagles," Weaver said.

Jim Adams • 612-673-7658