National study: Discharged lead ammo hinders eagle population growth, causes slow death

In August of 2018 this adult female golden eagle was captured and outfitted with a GPS transmitter. In early December of 2018, the eagle was recovered dead approximately 1.5 miles from where it nests in Yellowstone Park after movements ceased. The eagle was necropsied at the USGS National Wildlife Health Center where the cause of death was determined to be lead poisoning. Liver lead concentrations measured at 48 parts per million dry weight, which is in the range for lethal toxicity.

With its iconic white head and fierce gaze, the bald eagle is a symbol of American pride. But sometimes, that head bows in death due to leftover lead bullets.

The bald eagle is also the poster child for the effectiveness of the federal Endangered Species Act. In 1972, the U.S. banned the chemical DDT after biologists showed it was causing eagles’ eggs to be thin-shelled. Crushed eggs meant drastically fewer young survived. The population plummeted.

Since the ban, bald eagle populations have rebounded, and Missoulians can regularly see a few perched on cottonwood limbs above the Clark Fork River.

Even so, a new national study reveals the extent to which eagles and other raptors are still threatened by another compound: lead. Specifically, lead from hunting ammunition.

On Friday, a study published in the journal “Science,” detailed the results of an eight-year study of lead levels in golden and bald eagles. Biologists from 14 agencies, universities and companies – including Montana State University and NorthWestern Energy – joined those of the U.S. Geological Service in 38 states to sample more than 1,200 eagles, some of which, sadly, were already dead.

“This is the first study of lead poisoning of wildlife at a nationwide scale, and it demonstrates the unseen challenges facing these birds of prey. We now know more about how lead in our environment is negatively impacting North America’s eagles,” said Todd Katzner, USGS wildlife biologist and lead USGS author in a release. 

Lead poisoning typically occurs when an eagle eats an animal carcass or gut piles containing fragments of lead ammunition. Both eagle species are scavengers, using dead animals as a food source, but short-term lead exposure occurs more frequently in winter when live prey is harder to find and gut piles dot the land after hunting season.

About half of the eagles in the study showed repeated exposure to lead. If they don’t ingest enough to die within a short time, being repeatedly exposed to even small amounts of lead kills them eventually because the metal accumulates in bone tissue. This is why some of the dead birds that were sampled tended to be older.

These results are in keeping with those of smaller studies. A local study in the Bitterroot Valley found almost 90% of bald eagles sampled had elevated lead levels in their blood during the five-week rifle season.

Based upon this level of poisoning, biologists estimate that the annual population growth of bald eagles has slowed almost 5% and that of golden eagles is reduced by 1%. That jibes with a 28-year study, published a month ago, of bald eagles in seven Northeastern states that found lead ammunition had reduced population growth by 4% to 6%.

While 1% might not seem like much of a decrease, other studies are hinting that golden eagle populations aren’t doing as well as bald eagles to begin with. Counts by Hawk Watch International have documented that the number of golden eagles migrating over Montana have dropped by half since the 1990’s. A 1% annual decrease could have a bigger effect on the golden eagle population.

“The study’s modeling shows that lead reduces the rate of population growth for both of these protected species. That is not as impactful for bald eagles, since this endemic species population is growing at 10 percent per year across the U.S. In contrast, the golden eagle’s population is not as stable, and any additional mortality could tip it towards a decline,” said Brian Millsap, U.S. Fish and Wildlife Service National Raptor Coordinator and co-author.

While the study focused on eagles, they aren’t the only species affected by lead shot and fragments on the landscape. Wildlife rehabilitation centers regularly try to save the lives of various animals that ingest lead, from loons to foxes. In January, Montana Wild Wings Recovery Center of Kalispell reported the slow death of a trumpeter swan rescued from Flathead Lake.

Around the same time, four bald eagles died at the center. X-rays of the swan showed dozens of lead pellets in its gizzard. The swan likely sucked up the lead while feeding on the lake bottom. Lead shot has been banned for decades, but shot discharged years ago still persists in the environment.

“It was amazing the amount of lead this bird had consumed. It is sad to watch an animal die of lead poisoning – we hope people continue to consider using non-lead ammunition and lead free fishing tackle,” the Center said in a social media post.

Kate Stone, avian research director with the MPG Ranch near Florence, made a similar point about lead ammunition in January after watching an elk herd near Bell Crossing in the Bitterroot Valley. After hunters shot some of the elk on private land during shoulder season, Stone watched bald eagles gather for a potentially fatal feast.

“Multiple gut piles and what looks like at least one wounding loss have attracted the largest congregation of Bald Eagles I’ve ever seen in the Bitterroot. I think a count of 100 would be conservative. If the hunters involved used non-lead ammunition, then they’ve provided a valuable food offering to all of these eagles searching for sustenance in a time of low food availability. If they used lead ammunition, well, now we have a potential poisoning situation for 100 eagles as well as hundreds of crows, ravens, and magpies,” Stone wrote on social media.

Lead shot is banned, but lead bullets are not. Some hunting groups advocate for the use of copper bullets instead of lead. But while a few states have banned lead, switching to copper ammunition is still mostly voluntary.

“The big bonus here is that the lead problem is such an easy fix,” Stone told the Bitterroot Star in January. “Conversations about lead ammunition are not at all about stopping hunting. They’re about asking people to consider that they have choices, and that the choices they make really matter.”

As one of his last acts, former Fish and Wildlife Service Director Dan Ashe tried to get a national effort going by instituting a ban on lead ammunition and fishing tackle on all federal wildlife refuges. Former Interior Secretary Ryan Zinke overturned the ban a month later, at the urging of the National Rifle Association and some sportsmen’s groups.

Contact reporter Laura Lundquist at lundquist@missoulacurrent.com.