Lead in Bald Eagle Nestlings, 2006–2015

This article reports monitoring from 2006 through 2015. The health thresholds it cites are those in use when it was written.

Lead (Pb) is a toxic heavy metal that was widely used in the early 1900s in gasoline, paint and pesticides. Its use in the United States fell after the Clean Air Act of 1970 and later rules that began phasing lead out of paint in 1971 and out of most gasoline in 1990. Lead ammunition was banned for waterfowl hunting in 1991. Even so, lead is still found in some aviation fuels, ammunition for shooting sports and for hunting upland game such as deer and grouse, fishing tackle, batteries, tire weights and many other products.

Lead gets into the environment directly, through lost fishing tackle or chipped paint, and as products break down in the waste stream. People and animals that swallow it can suffer neurological damage. CDC set 5 micrograms of lead per deciliter of blood (μg/dL) as the level at which a child needs medical attention, and studies show that even levels below 2 μg/dL can cause behavioral and health problems in children. One review found that humans, birds and fish handle lead the same way they handle calcium, so much of what is known about lead poisoning in people applies to other animals too.

Why eagles

Bald eagles make good sentinels for contamination in waterways: they sit at the top of the food web, and their main prey is fish, a food people eat too. From 2006 through 2015, the National Park Service's Great Lakes Inventory and Monitoring Network sampled bald eagle nestlings at Apostle Islands National Lakeshore, the Mississippi National River and Recreation Area, and the St. Croix National Scenic Riverway. Biologists collected 367 feather samples from 2006 through 2015 and 188 whole-blood samples from 2010 through 2015.

Lead sinkers, shot and weights found in or just below bald eagle nests during 10 years of fieldwork for the monitoring program. National Park Service.

Where the lead comes from

Eagles can take in large doses of lead from spent ammunition, fishing tackle, or prey that already carries lead. One of the best-known sources of lead poisoning in bald eagles is bullet fragments in deer remains, but the nestlings were sampled in May and June, long after the fall deer season, so that route is unlikely here.

The biologists think the nestlings get lead mainly from eating fish. Fish remains turned up in 80% of nests, far more often than birds (30%) or mammals (25%). Fishing tackle was found in 2% of nests, including line with lead sinkers tangled in fish remains. Some exposure could also come from squirrels, turkeys or waterfowl carrying lead fragments or shot.

What the samples showed

Lead in feathers reflects long-term exposure, building up over weeks, while lead in blood reflects a recent meal.

• By place: lead in both feathers and blood was significantly higher at the Mississippi National River and Recreation Area than elsewhere. Levels were also high at Mississippi River Pools 3 and 4, followed by the lower St. Croix River, the south shore of Lake Superior, Apostle Islands and the upper St. Croix.
• Hot spots: high feather levels were clustered, with the highest in nests on the Mississippi near and just downstream of Pig's Eye Lake. High blood levels were more scattered, with the highest on the St. Croix River.
• Over time: lead in nestling breast feathers declined from 2006 through 2015, but blood lead did not show the same drop. The feather decline matches EPA data showing an 87% fall in the national average level of lead in air from 2000 to 2014, a fall largely credited to limits on lead in gasoline.

Highest lead concentration in eaglet feathers at three national park units and two nearby areas, 2006–2015. The yellow caution symbol marks the approximate location of Pig's Eye Lake. National Park Service.

Highest lead concentration in the whole blood of bald eagle nestlings, 2010–2015. National Park Service.

Lead concentrations in bald eagle nestling feathers by year, on a natural log scale. The median (line in each box) fell from 2006 through 2015. National Park Service.

A chronic source in the Twin Cities

Lead is declining overall but still building up in the food web, with some long-term local contamination, especially around Pig's Eye Lake. Many eagle territories on the Mississippi lie within metropolitan Minneapolis–St. Paul, near municipal and industrial wastewater plants, airports, refineries, power plants and heavy traffic, all possible sources of long-term lead exposure. Pig's Eye Lake sits next to the outlet of the metropolitan wastewater treatment plant and the former Pig's Eye Landfill, a Minnesota Superfund site, where the state had documented high lead levels in soil and water before the Park Service began monitoring. Some of the high feather readings may come from there.

Is it hurting the eagles?

No studies have looked at how lead directly affects bald eagle nestlings, and there are no guidelines for them. For adult eagles, the University of Minnesota Raptor Center treats blood lead of 20 to 60 μg/dL as subclinical exposure (contamination without signs of illness) and 61 to 120 μg/dL as clinical poisoning. One nestling had more than 26 μg/dL, above the adult subclinical threshold, and two others had 16 to 17 μg/dL. Twelve nestlings exceeded CDC's threshold for children.

The good news is that lead levels are falling overall, and the biologists found no sign that lead was reducing how many young the population raised. Nests along the Mississippi were significantly more productive than those on the south shore of Lake Superior or the upper St. Croix, even though Mississippi nestlings had some of the highest lead levels.

Sources

• National Park Service, Great Lakes Inventory and Monitoring Network, "Lead Contamination in Bald Eagle Nestlings." https://www.nps.gov/articles/lead-contamination-in-bald-eagle-nestlings.htm