Bald Eagle Research Reveals St. Croix Contaminants

Scientists briefly borrow baby eagles to study harmful substances.




2 minute read

A bald eagle carries a branch to its nest along the St. Croix River
A bald eagle carries a branch to its nest along the St. Croix River. (Photo by Gary Noren)

The bird that symbolizes America’s power and independence is today a key subject for scientists studying pollution in the St. Croix River.

Researchers recently visited bald eagle nests along the river to collect information that will tell them what kinds of harmful chemicals are moving through the food chain.

“We’re really using them because they’re at the top of the food chain,” Katie Nyberg, executive director of the Mississippi River Fund, told Minnesota Public Radio News. “So they just happen to be a great study subject to monitor these pollutants that tend to affect animals and humans throughout the environment.”

The young eagles studied also show what is in the immediate area of the nest, because their parents don’t travel far to find food, lead researcher Bill Route told MPR.

The scientists recently revisited a bald eagle nest near Marine on St. Croix — and other nests along the St. Croix River — to take samples. They have been monitoring nests along the river since 2006. The Country Messenger reported on this year’s effort:

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.

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Bald eagles were almost wiped out by pesticides and habitat loss during the last century, but have made a strong recovery in recent years. Deaths of young eagles first alerted scientists to the danger of the pesticide DDT in the 1950s and 1960s.

St. Croix canary

Anyone who spends time on the St. Croix knows an outing on the river today usually includes spotting at least one of the big and beautiful birds. More than 30 pairs nest along the river.

Samples from young eagles along the St. Croix and Mississippi Rivers and Lake Superior indicate the presence of leadmercury, the banned pesticide DDT, and other chemicals harmful to humans and wildlife.

Along the upper St. Croix, mercury is the biggest problem. Mercury from burning coal and other sources is deposited from the atmosphere and converted in wetlands to a toxic form that can easily find its way into the food chain. This is the same form of mercury which harms brain development in infants and children. The upper St. Croix is fed by water draining from many wetlands, leading to increased levels of this form of mercury.


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Bald Eagle Research Reveals St. Croix Contaminants