Written by Wendy Kaur, Cowboy State Daily
In the Big Horn Basin, golden eagles are the main predator, and their primary prey is cottontail rabbits. But a deadly disease has wiped out the rabbit population, which are a staple food for many different predators – so any reduction in their numbers would affect the entire ecosystem.
That’s according to Dr. Charles Preston, one of the world’s leading experts on raptors such as the golden eagle.
“Rabbit hemorrhagic disease virus (RHDV2) is a highly contagious disease transmitted between rabbits and hares,” Cowboy told State Daily. “In the ’80s and ’90s, this RhD virus was found in Europe, and it really devastated European rabbits and rabbits – and thus somehow destroyed the ecosystem.”
Preston is founder and chief curator of the Draper Museum of Natural History at Buffalo Bill Center in the West in Cody, and research associate at the Teton Raptor Center in Wilson. He explained that rabbit disease first appeared in the United States in 2018, and in Wyoming in late 2020. It kills up to 80% of infected animals and is rapidly spreading across North America.
Preston said the regular fluctuation in rabbit populations occurs every 6-8 years. The researchers expected a rebound in rabbit numbers in 2020, which should have led to a small increase in numbers of golden eagles.
“What we found is that the rabbits not only rebounded, but regressed from their previous lows,” Preston said. “And again in 2021, 2022, they really stayed in the tank.”
This year, Preston said, it has revealed the lowest abundance of rabbits they’ve seen since the study began in 2009. And it’s not just in cottonwood, the vulture’s primary food source.
“Here in our area, this virus affects both jackrabbits and cottontails – all types of cottontail,” he said. “In fact, it may even affect pica, close relatives of high-altitude rabbits.”
The effect on golden eagles
Along with staff and volunteers at the West Buffalo Bill Center in Cody, Preston has been studying the population of the Golden Eagle since 2009. This, he said, is the most dramatic population decline he’s observed.
“We now have 14 years of data showing cyclical fluctuations in golden eagle reproduction that are proportional to the rise and fall of cottontail abundance,” Preston said.
He explained that when food abundance is low, the egg-laying female golden eagle is not in the best physiological condition, and therefore may not lay eggs. “Sometimes, if they have enough nutrition, they can lay eggs, but they don’t have the energy, the nutrition to incubate the eggs, or – if the eggs hatch – to feed the young,” Preston said.
It does not pose a danger to humans
Preston stressed, however, that RHDV2 is in no way a danger to humans or other pets — simply to rabbits.
“The rabbits really take it on the chin,” he said.
Although golden eagles are versatile predators, Preston noted that even when cottontail rabbit numbers are low, they are still the preferred prey for birds of prey.
“Through these courses, we find that the Golden Eagle is still primarily focused on cotton,” he said. “Anywhere from 60 to 80% of the diet is made up of cotton — during the nesting season, at least.”
Preston said eagles would take other prey, such as crows, great-horned owls, wolves, spiny-horned sheep and even prairie dogs.
But the disease afflicted prairie dogs, too.
“There is an epidemic going through the prairie dog colonies,” Preston said. “So over the past several decades, prairie dog populations will probably be about 70% lower than their natural background would be.”
Expect a population recovery
In time, the rabbit population will return, Preston said, as those rabbits with immunity pass it on to their offspring. But this is in the future.
“Some rabbits may have normal immunity, just as we do with some diseases,” he said. “Over time, the population will rebound.”
But for now, the dramatic effect of RHDV2 is eliminating not only the rabbit population, but golden eagles and other predators that feed on rabbits.
He said there was no way of knowing the long-term effect on the golden eagle’s population.
“We expect them to expand their diet,” Preston said. “But it will not be enough to make up for the loss of cottons, probably. And so the breeding, as we have already seen, will remain very low.”
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