Every year, ecologists with climbing gear scramble to the tops of trees in Minnesota and Wisconsin to gather baby eagles from their nests. They're testing the eaglets for a variety of chemicals, including DDT, the one that nearly wiped out the eagle population decades ago.
By all accounts, the eagle population in this part of the world is doing very well. But the researchers are still finding plenty of chemicals in the blood of baby eagles. To find out more, we tagged along on one of those research expeditions.
National Park Service ecologist Bill Route and his crew are searching national park lands in Minnesota and Wisconsin, looking for eagle nests.
He points to a big old cottonwood tree, right at the edge of the Mississippi River in Lilydale Regional Park, across from Fort Snelling.
"It's that tree right there, kind of leaning over. It's hard to see, shrouded in all those leaves there," Route said.
Two budding biologists are using a cross-bow to shoot climbing ropes over a branch just above a nest.
"I think they have their first line fixed, so that means he's been successful at finding the right limb he wants to climb," Route says.
The climber hoists himself gracefully up the rope; then he disappears for awhile in the leaves.
The eagles in this popular park seem pretty calm about people. One soars in a wide circle above the nest, clucking mild disapproval at the intruders.
"Some pairs squawk a lot and make a lot of noise, but they never dive-bomb onto the climbers," said Route. "Other parents will just go to a tree nearby and just sit there and sulk at us."
The climber found three chicks in the nest, and now each one is tucked into a bag hanging from his belt.
The climbers carry the captives to dry land, where a dozen or so birders and park officials have gathered to watch.
They weigh the birds while they're still in the bags. Then they set the youngsters down on small canvas tarps to band them.
They take measurements that suggest these creatures are roughly seven weeks old. They're about the size of a newborn human baby, and they're quite calm.
They pant a little in the heat, and glare at their handlers with steady dark eyes. Their feathers are a little scruffy, and one of them smells like dead fish.
"Now we're going to do the most important sample, the blood sample," said Route.
The researchers are looking for six types of contaminants -- lead and mercury, DDT and dioxin, and a couple of newer chemicals.
DDT, the pesticide that was banned in the U.S. in 1972, has been declining steadily over the last 25 years. But Bill Route says it's still found occasionally in eagles' blood. DDT is still used in many places, including South America.
"It may be the prey they're getting from South America -- maybe migratory birds that are coming up, and they have [DDT] in their systems, they're preying on them; they're getting it in their systems."
Route says the ban on lead pellets in shotguns has helped reduce lead poisoning, but eagles are still poisoned by lead from fishing tackle.
And then there are the new chemicals, like flame retardants. Route says in the first part of this decade, the flame retardant chemicals found in eagles' blood doubled. But recently Europe and some states have banned certain formulations, and those chemicals immediately began to decline.
"But there's 170 formulations of flame retardants, so it's pretty hard for us to keep up with," he said.
They're also checking for perfluorinated chemicals, like the stain-resistant and heat-resistant products once made by 3M. Traces of those products spiked in eagles five years ago, but they've been declining quickly since 3M stopped making them.
These new chemicals don't have the outright lethal effect that DDT had on eagle populations. But Bill Route says they may have subtle effects, especially because there's so many of them.
"So it could be these eagles or other animals -- or ourselves -- just aren't functioning quite the way we could be if it wasn't that we have a number of chemicals in our system affecting our neurology, affecting our endocrine system," said Route. "And I think there's lot of linkages that are tantalizing but not exactly smoking guns yet."
The population of eagles along the Mississippi River in the Twin Cities area is growing more than 10 percent each year. And that includes the three baby eagles returned to their nest after their adventure with the scientists.