The white pine swayed in the early evening winds as biologist Chris DeSorbo carefully, cautiously positioned himself on the edge of a massive nest built into the tree’s crown.

It had taken DeSorbo about 10 minutes to climb the 75-foot tree. Now came the more challenging task of getting a feisty, 7-week-old bald eagle – already not pleased about this intrusion on his family life – into a canvas sack.

Twenty minutes and a lot of coaxing later, the eaglet was lowered by rope into the hands of DeSorbo’s partner on the ground.

“He’s a big boy,” Chris Niven said as he attached metal identification bands to the unhappy eaglet’s legs and took blood and feather samples. The nearly full-sized bird was then sent back up to DeSorbo for re-nesting.

The fact that the two were on their third eaglet that day, and approaching their 30th hatchling in a several-week span, speaks to the dramatic recovery of Maine’s bald eagle population.

Maine is home to an estimated 400-plus pairs of breeding bald eagles, according to aerial surveys. DeSorbo and Niven, who work for the BioDiversity Research Institute in Gorham, as well as a second team of biologists were only able to visit about 60 of those nests this spring as part of a study on pollution in eagle habitats.

Thirty years ago, a biologist could have scoured all of Maine for months and found fewer hatchlings than DeSorbo handled in two afternoons.

Today, Maine accounts for more than 90 percent of all breeding pairs of the national bird in New England.

It’s a point of pride for the many people who have been working to repopulate the state with bald eagles from a low of just 21 breeding pairs in 1967. Nationwide, eagle populations have rebounded to the point that federal officials are expected to remove the bird from the list of threatened species.

“Everybody looks to Maine to be the stronghold in the northeast, and we take that role very seriously,” said Charlie Todd, the head eagle biologist with the Maine Department of Inland Fisheries and Wildlife.

Maine biologists take the job seriously because they realize that Maine’s eagle population, while robust compared to other northeastern states, is not entirely clear of the chemical dangers that nearly drove the bird to extinction in the lower 48.

Researchers with the U.S. Fish and Wildlife Service, Maine DIFW, FPL Energy-Maine Hydro and the Gorham-based BioDiversity Research Institute are in the middle of a multiyear study to determine how much pollution is affecting Maine’s eagle population.

Earlier this year, Steve Mierzykowski with the USFWS removed the livers of more than a dozen dead eagles for analysis for a variety of contaminants, including mercury, dioxin and the family of chlorinated organic pesticides – such as DDT – largely blamed for the dramatic population decline in the 1960s and 1970s.

That same research project brought Niven, DeSorbo and Mierzykowski to the eagle’s nest on a Penobscot County pond earlier this month. Instead of liver samples, the group is collecting blood samples from eagle nestlings all over the state to compare mercury levels in different habitats.

DeSorbo, who is using the mercury program as his master’s thesis from Antioch New England Graduate School in Keene, N.H., suited up in a rock climber’s ropes, harnesses and helmet before beginning his ascent of the pine.

Contrary to their reputations as fierce predators, adult eagles generally do not bother the climbers, even when they remove the eaglets from the nest. Instead, they typically circle overhead sounding an alarm or watch warily from a nearby perch.

“Actually, they are quite passive birds,” DeSorbo, a research biologist and director of BRI’s raptor program, explained before heading up the tree. “Ospreys will hit, and a lot of other raptors will hit you. But I haven’t had an adult eagle come at me.”

That doesn’t always mean that the eaglets go willingly, however.

Climbers from the two research crews must time their visits within a narrow two to three week window when the birds are old enough to be handled but too young to attempt flight, a potentially fatal misstep. They also must be wary of strong winds that could sweep an eaglet out of the nest when it spreads its wings to intimidate the climbers.

“They are both super careful because there is no room for error,” Mierzykowski said of the climbers as he watched from below while DeSorbo made his way around the nest.

This particular eaglet appeared healthy, both physically and spiritually, as he tried to bite Niven, Mierzykowski and a camera lens.

Weighing in at nearly 10 pounds, the young eagle was covered head-to-talon with dark plumage and appeared well fed. Bald eagles do not generally grow their trademark white head and tail feathers until reaching sexual maturity around age 5.

DeSorbo, meanwhile, collected a bag full of prey remains in the nest and searched for any unhatched eggs while waiting for Niven to send the bird back up.

Todd, DIFW’s eagle biologist, said mercury contamination seems to be a larger problem among eagles who nest on inland lakes and ponds rather than in coastal areas. At the same time, the number of breeding pairs with inland nests recently surpassed the number of pairs on the coast, Todd said.

The federal government is widely expected to remove the bald eagle from the list of threatened species either later this year or in 2007. Eagles will remain protected from most harm under the federal Bald and Golden Eagle Protection Act.

Maine may opt to keep the eagle’s “threatened” designation on the state’s own species list. Todd said the state’s de-listing decision will likely hinge on a variety of factors, including how many nesting sites have been formally protected as well as the results of the mercury and contaminant study.

While Maine’s eagle population has consistently grown by about 8 percent annually, the birds are returning in even larger numbers in other states, especially around the Chesapeake Bay and the Great Lakes.

Eagles also are not equally dispersed throughout Maine. York County, for instance, got its first pair of breeding eagles just last year.

Todd said he believes there is still plenty of room for population growth in the state.

“We are not looking for sheer numbers of birds but also for a good distribution,” he said.