Approximately 100 feet in the air a large nest located along Judd Street in Marine on St. Croix is home to bald eagles. On June 10, researchers collected information on the eaglets, including sampling blood to identify any contaminants as well as getting measurements of the birds. Photo by James Sprickler

Above the banks of the St. Croix River lies a prime breeding spot for bald eagles. The national bird is typically found near large bodies of open water with an abundant food supply and large trees for nesting. 

Bald eagles are also a good sentinel species for monitoring environmental contaminants, says Bill Route, program manager andecologist with the National Park Service’s Great Lakes Inventory & Monitoring Network (GLKN). 

Since 2006, scientists have been collecting data from nestlings – usually between five- and nine-weeks-old – at various locations throughout the St. Croix National Scenic Riverway. To accomplish this task, a skilled climber will ascend the tree to hand capture any nestlings that are in the bald eagle’s nest. Once on the ground, plucked breast feathers indicate what inorganic metals are present. Blood will be sampled and tested for chemical contaminants. Measurements are taken to determine the bird’s age and sex. Each nestling is banded around the leg. 

Route said it could take several months before the information is analyzed. There are over 85,000 chemicals in the environment today and approximately 2,000 are introduced each year. GLKN is monitoring a handful of chemicals that are contaminating aquatic systems like the St. Croix River. The known toxins include heavy metals, mercury and lead; banned PCBs and DDT along with its derivatives, DDE and DDD; and emerging chemicals, PBDEs (flame retardants) and PFCs (stain and water resistant chemicals). 

The bald eagles numbers dwindled so low in the mid-20th century that the bird was recognized by the federal government as an endangered species, but by 1995, was reclassified as “threatened” and de-listed in 2007. 

Route explained that the Minnesota Department of Natural Resources (DNR) tracked a nest in Marine on St. Croix by aerial survey, which is how it came to be on his radar. As the population increased so did the abundance of nests. Now the DNR in Minnesota is only doing fly-overs every five years, he said. 

“We have now started monitoring it ourselves,” Route said of the Marine nest. 

However, the Wisconsin DNR flies over yearly. 

“We’ve asked them to peek over the border and tell us if it’s active or not,” he said. 

Almost a decade ago, it was estimated there were more than 1,300 active nests in Minnesota. Only two states – Florida and Alaska – have greater nesting populations of bald eagles than Minnesota.

The first year of sampling in 2006 failed to produce any results, Route noted. Now the GLKN has six years worth of data, which he said is a good baseline. 

“From this point forward, we’ll sample two years on and two years off,” Route said.

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