A new study published in The Journal of Wildlife Management focuses on the negative impact of feral horse populations on sage grouse. Researchers at the University of Wyoming (U-WY) found that feral horses have decimated critical sagebrush-steppe habitat, which the birds rely upon, while also affecting the sage grouse nesting, hatching, and young bird survival, Wyofile reports.

“Very clearly, our results suggest that if we don’t go over [horse population goals], then sage grouse nesting, brood-rearing, and juvenile survival is going to perform much higher,” said Jeff Beck, a U-WY ecologist and lead researcher for the study. “Where horses are sharing habitat with sage grouse, it’s really important to maintain horse numbers that don’t exceed the maximum [population goal].”

Using what the Bureau of Land Management considers the maximum “appropriate management level” of horses as a gauge for the study, researchers monitored about 1,000 sage grouse via GPS tracking in southern and central Wyoming. They looked at nest success, including brood, juvenile, and adult survival, both where – and where not – feral horses lived. They discovered how far away from that “appropriate management level” feral horse populations were (despite regular roundups) and the harmful impact of these burgeoning populations on sage grouse.

For example, nest survival decreased by 4% for sage grouse that lived within areas where horses were at objective versus those that lived where horse numbers were three times over that – those areas dipped in nest survival rates by 8%, according to the study. Further, sage grouse chick survival rates decreased by 18% in areas where feral horses were triple over “appropriate management level,” and juvenile sage grouse were at a similar level in those areas.

Fortunately, for birds that reached adult stages, the number of feral horses didn’t seem to have the same impact. Regardless, feral horse populations need to be in check for the continued survival of an already imperiled species.

“We’re not saying, ‘Get rid of all the horses,’” said Beck. “Our message is to manage it. If you manage it not to exceed that [goal], then sage grouse that share habitat with horses can continue to do well.”