Unraveling the Mystery of Golden Eagle Nests

A juvenile eagle spreads its wings. Photo by BJ Worth – WingsInNature.org

In Glacier National Park, where the air is crisp and the mountains are steep, a remarkable bird can be found soaring on thermals across the expanse of blue sky. The golden eagle is a giant of the sky, the top avian predator. The regal presence of this raptor, with its wingspan as large as a person, leaves observers looking for some comparison. “They are the grizzly bears of the sky” suggests Lisa Bate, a Glacier National Park wildlife biologist.

In the mid-1990s biologists knew of 49 golden eagle nests in Glacier National Park, but by 2020 there were no known active nests in the park. In 2021, there was just one active nest found, which subsequently failed. This year, 2023, Bate and her team have located 7 active nests! With the help of citizen scientists, they will monitor these nests until the chicks fledge (develop wings large enough to fly) or until, sadly, the nest fails.

Ava Johnson is the wildlife technician assigned to the golden eagle nesting monitoring project. She and the team started searching for active nests back in April, in preparation for the citizen scientists arrival in June. The team worked with Glacier National Park Geographic Information System (GIS) Specialists to locate prime golden eagle habitat and historic nest locations. Teams would then hike to these locations, find an observation point, set up spotting scopes, bring out the binoculars, and begin scanning the mountain cliffs in hopes of finding a golden eagle nest.

Golden eagles in Glacier National Park build their nests exclusively on mountain cliff ledges with overhangs and in rock crevices. They look for open ground below so they can hunt for marmots, ground squirrels, ptarmigan, and grouse. Their stick nests are typically 10-12 feet wide and up to 15 feet deep to accommodate their 6-foot wingspan.

A golden eagle couple usually has multiple nests in a territory and will move between these nests from year to year. They will bring new sticks to the nest of choice in preparation for the chicks, so looking for green on a nest is a great way to determine if it is active. A nest with all gray sticks has likely been inactive for over 2 years. A nest with red needles was likely active 1-2 years ago. If you see a nest with new green sticks, bingo, you have a high chance this nest is active in the current year.

When golden eagles breed, they lay only 2 eggs and both eggs may not hatch or survive. It is the job of the parents to incubate the eggs, protect the eggs and chicks from the harsh weather of Glacier, and feed the chicks once they hatch. It takes 2-3 months for the chicks to develop, so teams are prepared to monitor the nests until about the third week of August.

“It is so cool to watch these chicks develop over 2-3 months in the nest!” Bate added, “And if you are lucky enough to see one fledge that’s incredible … they go from barely hanging onto the side of the nest and before you know it, they are flying and begging for food.” Next up for the successfully fledged chicks is the fall migration that takes place in October, the peak of which is from October 10th to 17th.

Lisa and her team plan to monitor these 7 nests through fledging and hope to continue to secure funding to monitor nests in coming years. If these avian predators were to disappear from the park, the whole ecosystem is in trouble. Monitoring golden eagle nests will help determine the overall health of Glacier National Park.

In many other parts of the world golden eagles are doing well, but in our area, lead poisoning is a huge factor in killing golden eagles. While hunting in Glacier is illegal, golden eagles will fly out of park boundaries and eat the carcasses of ground squirrels and other animals that were shot with lead bullets. Nest monitoring will help scientists track the prevalence of botflies and avian flu (not yet found in GNP). It is because of people like Lisa Bate, her team of biologists, citizen scientists, and donors like you, that golden eagles are being monitored so we can better understand the success rate of breeding eagles in Glacier National Park.

“This work wouldn’t happen if we didn’t have donations from the Conservancy and all the supporters,” Lisa Bate explained, “I don’t have a wildlife budget except through grants… So if it wasn’t for the Glacier National Park Conservancy and its donors almost none of my wildlife research and programs and inventory monitoring programs would happen.”