In July 2019 a game warden in Bridgton, Maine, got an unusual call: A bald eagle was floating lifeless in a lake. At the time, biologists suspected the animal might have been shot or poisoned by lead fishing tackle—all too common causes of death for wild birds.
According to D’Auria, a dead loon chick was found nearby, suggesting a defensive loon parent gored the eagle as it attacked the loon’s nest. This phenomenon is on the rise in New England, as bald eagles continue to bounce back from near extinction in the 1970s, she says. (Learn how a national symbol bounced back.)
Loons and eagles are also top predators in Highland Lake, competing for valuable territory.
While loons appear serene and peaceful, the waterbirds can be savage, attacking everything from Canada geese to redhead ducks to, most often, other loons.
The catch is that until very recently there probably just weren’t enough bald eagles left for scientists to witness such battles. Since being removed from the Endangered Species List in 2007, the U.S. symbol now numbers in the hundreds of thousands nationwide; there are more than 700 nesting pairs in Maine.
The incident shows how much we have to learn about the natural behaviors of formerly endangered species, experts say.
Violence of the loons
Rather than duke it out at the surface, D’Auria says a loon will dive underwater and then rocket out “like a torpedo” to stab its opponent, which is usually a rival loon.
“It’s a common part of their contesting territories with each other,” she says. ”Sometimes the injured loon can recover from it, and occasionally they don’t.”
In fact, Cooley says he's seen a loon chest bone riddled with holes. “Over half of the loon mortalities that we examine show healed puncture wounds like this eagle sustained,” he says. (Read about a bald eagle rescued on the Fourth of July.)
Loons can also be extremely long-lived, with one banded bird in New Hampshire defending the same territory for at least 26 years.
For this reason, "they’re invested in their lake. It’s their little kingdom,” says Cooley.
A bird-eat-bird world
At over 10 pounds, adult loons are generally too large for a bald eagle to kill and wing back to its nest.
However, loon chicks are perfect prey for bald eagles, and scientists are only recently beginning to document how the return of eagles might be affecting loon populations in New England.
One study led by Cooley found that loon nests seemed to fail more often when they were located near bald eagles.
“There’s a balance,” he says, by email. “Eagles need to eat, and loons will defend their chicks as best they can.”
The good news is Vermont’s loon populations have been increasing or remained steady for the last 20 years. Loons are also doing well in Maine, home to about 70 percent of the population in the U.S. Northeast, says D’Auria.
So while neither loon nor bald eagles seem to be in danger of driving the other to extinction, it does seem as if the two species are recalibrating back to how things used to be, Cooley says.
There are plenty of other examples: When conservation efforts enabled gray seals to return to their native territory in Cape Cod, great white sharks followed closely behind. And in the mid-90s, when the National Park Service reintroduced wolves to Yellowstone, it set off a cascade of ecosystem changes for everything from elk and coyotes to aspen and willow trees—changes scientists continue to puzzle over. (Read more about the impact predators make in Yellowstone National Park.)
It’s just that this time, the loon killed a bird that most Americans feel strongly about protecting.
But Cooley says this event, sad though it was for the eagle, is the goal of species recovery.
“We want natural problems like this to replace the human-caused problems, like lead fishing tackle as a source of mortality,” he says.
“You know, we’re living for the day when eagles are the worst thing that loons have to deal with.”