Study of golden eagles in Bighorn Basin revealing details on apex predator 

Golden eagles living in Wyoming’s northern Bighorn Basin are so dependent on cottontail rabbits as a food source that bunny abundance is a big driver of eagle reproductive success.

“The biggest surprise, or at least the most dramatic and profound result, is that production from eagles may vary dramatically from year to year,” said Chuck Preston, senior curator of natural science at the Draper Natural History Museum in Cody, Wyo. “The primary driver of that is rabbit abundance. So we’re trying to define rabbit cycles in this area.”

In other areas of the West, jackrabbits – whose populations cycle about every 10 years – were known to be a key food for golden eagles, but that’s not the case in the Bighorn Basin.

Long-term study

Preston has been conducting a study of golden eagles across an area of about 1,500 square miles of sagebrush steppe habitat in northwestern Wyoming since 2009. It’s a unique place – a high desert of sagebrush, sandstone and rolling hills mixed in with irrigated farmland and framed by the Carter and Beartooth mountains to the west, and the Bighorn Mountains to the east.

“The area has some of the best quality sagebrush steppe habitat,” he said. “We just wanted to get a baseline of eagles. So even after we stop studying, people can have a baseline to go from in determining golden eagle populations.”

Fluctuations in the eagle populations can be good barometers of environmental and land use changes, he noted.

Species of interest

Golden eagles are of interest to Preston because they are an apex predator in the basin. It’s easy to understand why when you consider that a large female may have a wingspan of up to 7.5 feet and weigh 10 to 14 pounds. The dark brown bird with flecks of gold around its neck is such a deadly hunter because it is armed with eight 2- to 3-inch long talons. When deploying its talons, the bird has a grip more than 10 times stronger than a human’s, capable of exerting 750 pounds per square inch of pressure.

Because of its hunting prowess, ranchers targeted the birds and between 1940 and 1960 an estimated 20,000 golden eagles were shot from planes before the federal government declared them a protected species.

Despite such protection, their numbers have declined since then, largely because of a loss of habitat from human development. Other factors contributing to fewer golden eagles have been fires, exotic plant species and speculation that some of the carrion eaters have died from lead poisoning from dining on animals shot with lead bullets. A Montana study published in 2009 showed 58 percent of 42 golden eagles captured in the fall had elevated lead levels in their blood.

Windmill threat

“It’s amazing how this thing snowballed,” Preston said. “Golden eagles have attracted so much more attention because of wind turbine development across the West.”

Largely because of this more recent threat – one study Preston referred to showed that 79 golden eagles were killed at 32 wind farms in 10 states between 1997 and 2012 – in 2013 the U.S. Fish and Wildlife Service created a western golden eagle conservation team to address concerns and find solutions to “move the conservation needle.” As part of that, the agency sought out already ongoing studies of golden eagles to get a better understanding of the landscapes eagles were occupying, their migration routes and timing, as well as reproduction in hopes of identifying limiting factors.

The idea is to head off any dramatic decline in the birds’ population, like the one that saw the golden eagle’s cousin, the bald eagle, placed on the endangered species list. The bald eagle, along with some other raptor species, was decimated by the widely used chemical DDT, which weakened the bird’s eggshells. After the chemical was banned, bald eagle numbers eventually rebounded and the bird was taken off the endangered species list.

Preston’s work caught the eye of the USFWS because he has collected several years of data on golden eagle nesting and reproduction in the Bighorn Basin.

“In the study area we have identified 70 nest territories that have been occupied by eagles at least one year since 2009,” Preston said.

Nest data

By visiting those nests, Preston and his crew of volunteers has collected 800 prey remains from 20 nests over the last several years. The eagles may key on cottontails, because they are plentiful, but the big birds also dine on antelope fawns, other birds like magpies and even other predator birds like great horned owls.

“We were surprised at the number of great horned owls,” Preston said. “They’re a pretty impressive predator on their own, but they’re not the apex predator when they are in golden eagle territory.”

Great horned owls are also competitors with eagles for food, since they also dine on rabbits, as well as small rodents, and may even eat an occasional eaglet.

Golden eagles that occupy other areas often have a greater variety of prey than those that inhabit the Bighorn Basin, Preston said, allowing them to shift to other species when one is less available. That’s not the case for the rabbit-dependent eagles of Wyoming, though.

One bird that Preston expected to find the remains of in the Wyoming golden eagle nests was sage grouse.

“We went into it assuming, because it was dogma out there, that eagles take a lot of sage grouse,” Preston said. “But we’re not seeing that at all.”

Bird ID

Last year, Preston and his crew helped the USFWS place four GPS transmitters on young eagles to determine where they disperse to during the three to five years that they leave their birthplace and before they get old enough to breed themselves.

So far, three of the four GPSed birds have stayed in the general area of the study, moving about 40 to 60 miles.

Since 2009, Preston has also placed leg bands and wing bands on fledglings to help identify them.

“We’re hoping to find some nesting birds that we banded four to five years ago” this year, he said.

Preston is also interested in seeing how oil and gas development in the region, and even home construction, can affect golden eagles. Other studies have already shown that golden eagles do better in areas with fewer roads and traffic – they are a bird of large, open spaces.

“For eagles the best habitat is grassy, shrubby and even rocky areas, which makes sense because that’s good habitat for rabbits, but you need the grassy areas for the eagles to capture rabbits,” Preston said.

Interestingly, some oil and gas outbuildings, in areas without high human traffic, can actually provide rabbits with habitat to avoid predation and therefore increase their populations, he said.

“We’re getting a greater understanding of how the system works,” Preston said.

“One of the take-home messages from all of this is: Eagles are long-lived, so there’s a greater chance there will be a large variability from year to year. If you want to get a sense of baseline numbers, you have to watch over them for a long period.”

As the Wyoming landscape continues to be developed and change, habitat for golden eagles is likely to diminish, and as a result, their populations could fall even more.

Preston said he’s hopeful that in Wyoming and Montana, development can be undertaken with an eye toward what’s best for the landscape and the wildlife that occupies it, like the golden eagle.