Originally published in Points West magazine Fall/Winter 2017
Dispatches from the Field: The Eagles of Rattlesnake Gulch
By Charles R. Preston, PhD
In 2008, Dr. Charles R. Preston, Willis McDonald IV Senior Curator of the Draper Natural History Museum, began a study of golden eagles in the Bighorn Basin of Wyoming. Since 2012, he’s shared updates with Points West readers as he and his crew push toward the ten-year mark of the study. Here is the latest:
Rattlesnake Gulch 2017
We almost failed to recognize our favorite parking site as Draper Museum Research Assistant Nate Horton eased the Yukon along the winding dirt road south of the big golden eagle nest. The winter’s deep, slow-melting snowpack had saturated the ground so thoroughly that tall grasses and pioneering weeds transformed this usually-barren parking spot into a miniature jungle.
The nest was located high on a cliff face about 500 yards from the dirt road. It was protected from above by a large rock overhang, but we could clearly see the nest from our position. We assumed that the same adult pair of eagles—who had occupied this site each year since at least 2008—were back this year. Pilot and project consultant Richard Jones had reported seeing fresh greenery on the nest, and two adults perched nearby when he conducted aerial surveys in early March. In early April, Bud and Dale Schrickling, two of our Golden Eagle Posse citizen science volunteers, reported two young eaglets already in the nest.
These were the first eaglets to hatch among the thirty-plus golden eagle nests we monitored this year. It was May 12, and Nate, seasonal assistant and project photographer Nick Ciaravella, and I were here to examine, measure, and band the two nestling eagles on this sunny, unseasonably warm morning. As the nestlings’ parents watched from a distance, Nick rappelled deftly into the nest. He placed hoods on the youngsters to calm them, placed them in protective carriers, and lowered them to Nate and me below.
Carefully avoiding the sharp, powerful talons, we examined the young birds and estimated that both were about four weeks of age. Their parents had apparently just delivered a fresh rabbit for breakfast. They had full crops, and each weighed more than eight pounds. We secured a series of several standard measurements and placed a small, uniquely-numbered leg band on each bird. Females are generally larger than males of the same age and have proportionately larger feet. We determined from our measurements that we were handling one male and one female, and they appeared to be in excellent health.
We returned the birds to the nest, loaded our backpacks, and hiked back to our vehicle. We then watched as one of the adults returned to the nest to tend to the nestlings with their new ankle bracelets. As we drove away, I thought about the many times I’d visited this nest before, and how this family of eagles epitomized our long-term golden eagle study in the Bighorn Basin.
The first visit to Rattlesnake Gulch
I first visited this nest site in August 2008. I was scouting likely golden eagle nesting areas in the northwestern region of the Bighorn Basin before submitting research grant proposals to various agencies and foundations. My research objectives were to develop baseline measurements for golden eagle nest distribution, reproductive activity, diet, and prey abundance in advance of inevitable habitat changes expected in the native sagebrush-steppe landscapes in the region. (The study has been described in several past Points West issues, e.g. Spring 2015, Fall 2012.) In that first year, I reviewed historical accounts of nesting areas, and Richard Jones provided me with several locations he had discovered from the air.
I struck out on a Friday afternoon to see if I could locate a nest site Richard had discovered in the northern portion of the study area. I had no trouble finding the nest, thanks to a broad wash of white on the sandstone wall behind the nest. The whitewash was created by perhaps many decades of eagle defecation splattered on the surrounding rocks. The young eagles had already left the nest, and I did not detect any adult or juvenile eagles nearby.
I loaded my camera, hand-held GPS (Global Positioning System) unit, water bottle, some polyethylene bags, and other gear into my daypack, and set out to get a closer look at the nest. I navigated the first obstacle, a deep, mostly dry creek bed bordered by thick shrubs. After making my way through the tangled shrubs and sinking knee-deep in a muddy pool, I headed for a long, rocky gulch passing beneath the nest.
Dripping with sweat, I sat down on a rock outcrop for a quick drink. While I was pulling my water bottle from the daypack, I caught a movement in the shadow under a large boulder about ten feet in front of me. It was a two-foot-long prairie rattlesnake slithering deeper into the shade! Making a mental note to steer clear of shady areas that might hold more snakes, I proceeded along the gulch toward the nest. On the way, I spotted two more rattlesnakes. From that point on, this place became Rattlesnake Gulch in my field notes.
The nest site was perched high along the rim of the gulch, so I had to scramble up the steep, rugged slope to find an observation point that looked down on the nest. Once above the nest, I lay down on an overhanging rock to get a good view of the nest. It was strewn with the remains of prey eaten by the recently fledged eagles. Cottontail rabbit legs, ears, and other body parts dominated the scene, but there were also raven’s wings and portions of two large snake skeletons. I decided to collect and identify as many of the prey remains as possible.
I carefully eased out on my stomach along the rock outcrop, dangled my legs over the edge, and gently dropped the eight feet onto the ledge that held the nest. Once on the ledge, I took several photographs and collected two bags full of prey remains to identify later in the Draper Museum laboratory. It was now late afternoon, and I was aware of dark clouds closing in from the south. When lightning began bursting from those clouds, it occurred to me that it was a good time to head for my vehicle—and that’s when I also realized that I had overlooked something important.
I was on a thin ledge about twenty-five feet above the floor of Rattlesnake Gulch, and the rock outcrop I had dropped down from was too far above me to reach. I was stranded! My cell phone had no service, but I was loath to call for help anyway. I sat down and assessed the situation for about fifteen minutes. There was a relatively smooth slope peppered with sagebrush and greasewood shrubs below me, but I would have to clear a large boulder to reach the slope and try to slide and roll the rest of the way down—and lightning was coming closer. With a deep breath and mighty leap, I cleared the large boulder, and began crashing, rather than sliding, down the slope. With each sagebrush I smashed into, I envisioned a startled and angry rattler!
After perhaps five seconds that seemed like an hour, I came to a stop against a large greasewood shrub at the bottom of the gulch. As rain began to pour from the dark clouds above, I picked myself up, checked for broken bones (miraculously, there were none), examined my camera and other contents of my daypack, and limped to my vehicle—a bit worse for wear, but also a little wiser.
I turned around to take a last look at the nest. A pair of adult golden eagles, perched side by side above the nest, watched my unsteady departure; I imagined them laughing at my escapade. Since that first encounter, I’ve gotten to know the eagles of Rattlesnake Gulch, their diet and nesting habits, and many of their offspring very well. Their story is a microcosm of the larger story of golden eagle nesting ecology and wildlife ecology throughout our study area.
Officially launching the study
We began monitoring the eagles of Rattlesnake Gulch in earnest in 2009. Golden eagles form pair bonds and begin breeding when they are four or five years old. They tend to mate for life and, except in the case of significant disturbance, tend to occupy the same breeding territory each year. They may maintain several alternative nests within the same territory. The eagles of Rattlesnake Gulch have occupied their breeding territory each year of our study, and we have only observed them using one of their other nests.
During the past nine years, the eagles of Rattlesnake Gulch have successfully produced a total of ten fledglings. A fledgling is a young bird that is old enough to leave the nest and survive, usually with some extended help from their parents. Thus, the average annual reproductive rate of this pair of eagles has been 1.1 offspring produced per year of our study. This is slightly higher than the average annual reproductive rate (0.83) of all eagle territories we’ve monitored during the last nine years.
The Rattlesnake Gulch parents have failed to produce offspring during two years of our study. These were years when our nighttime roadside surveys showed that cottontail rabbit abundance was especially low. During years when cottontail numbers were high, the Rattlesnake Gulch eagles consistently produced two offspring, and in average cottontail years, this pair produced one offspring. We have found that cottontails are by far the most important prey in a golden eagle’s nesting diet throughout our study area, and that when cottontails decline, so does golden eagle reproduction. In the case of our Rattlesnake Gulch eagles, cottontails have comprised 78 percent of the nesting diet.
Other prey remains we’ve retrieved from the nest include white-tailed jackrabbits, pronghorn fawns, a few kangaroo rats and bushytailed woodrats, two great horned owls, two greater sage grouse, some common ravens and other birds, and a few rattlesnakes and bullsnakes. Nesting eagle diets from throughout our study area are similar to the Rattlesnake Gulch eagles.
Catching up with our subjects
We’d like to keep track of as many eagle offspring as we can to understand eagle movements, habitat use, and population turnover. Since 2009, we’ve banded more than sixty young eagles, including nine from the Rattlesnake Gulch family. We worked with the U.S. Fish & Wildlife Service in 2014 to attach satellite transmitters (tags) to four young eagles, including one female from Rattlesnake Gulch. We still receive locality information from the Rattlesnake Gulch eagle and two tagged eagles from other families. Unfortunately, we found the lifeless carcass of the fourth eagle south and west of Meeteetse, about fifty miles from where she hatched. She was a year old when she flew near the nest of another pair of golden eagles and was apparently killed by one of those aggressive birds.
Our satellite-tagged Rattlesnake Gulch female was a big (more than 10 pounds), healthy, and feisty fledgling when we captured her a few days after she left the nest in 2014. She stayed near her parents, foraging around Rattlesnake Gulch until the winter of 2014–2015, when she left the area. Since that time, she’s spent most of her time in the vast open sagebrush and desert shrub flatlands between Casper and Worland, Wyoming. She’s now three years old and may form a pair bond and begin producing her own young as early as next year. The other two tagged eagles still alive initially wandered north to central Montana, but have spent most of their time back in the Cody area during the last two years. They, too, could begin nesting next year.
Our study has revealed new information about the specialized diet of the usually versatile golden eagle and the impacts of primary prey decline on eagle reproduction. We’ve also contributed information on blood toxicology, parasites, and disease in a series of continent-wide golden eagle studies. Together, these studies are crucial in helping minimize or mitigate negative impacts of rapidly increasing habitat changes in western North America.
We’ve also learned a great deal about dispersal and mortality of young eagles fledged in the Bighorn Basin, and the ecological dynamics of sagebrush-steppe environments in Greater Yellowstone and the American West. Some of these findings are detailed in two scientific articles scheduled to be published in a special edition of the international Journal of Raptor Research in late 2017. Several more articles and a book are in preparation. The plan is for our study to continue at least through 2018, when we review ten years of data and determine the most important and fruitful paths for future research.
The rich wildlife heritage of the American West is renowned worldwide. We rely on large sample sizes and population-level statistics to understand wildlife ecology on a large scale in the American West. However, we also gain a different level of understanding and important insights from more intimate observations on individuals and family groups like the eagles of Rattlesnake Gulch. There’s truly nothing like seeing those “monarchs of the skies” up close and personal.
Dr. Charles R. Preston is the Willis McDonald IV Senior Curator of Natural Science and Founding Curator-in-Charge of the Draper Natural History Museum and its Draper Museum Raptor Experience. Prior to that, his career path included a tenured professorship at the University of Arkansas, Little Rock; Zoology Department Chairman at Denver Museum of Nature and Science; and adjunct faculty appointments at the University of Colorado (Boulder and Denver); University of Denver; and the University of Arkansas, Fayetteville. Preston currently focuses on human dimensions of wildlife management and conservation in North America, especially the Greater Yellowstone region and the American West, as he studies raptors and predator-prey dynamics, informal science education in society, and the role of scientists as public educators.
Monarch of the Skies
In June 2018, the Buffalo Bill Center of the West unveils a new permanent exhibit in the lower level of the Draper Natural History Museum. Monarch of the Skies: The Golden Eagle in Greater Yellowstone and the American West showcases the long-term Draper Museum study of golden eagles in the Bighorn Basin and explores the natural history, ecology, and conservation of golden eagles and sagebrush-steppe environments throughout western North America. The exhibit also highlights the rich association between Plains Indian cultures and the golden eagle. Find more information about Monarch of the Skies in this Points West Online article, first published in the summer 2018 issue of Points West magazine.