Mesa Verde announced the beginning of a public comment period Thursday that could lead to a roundup of the horses later this year.
The national park in southwest Colorado has 100 to 150 horses in and near the park's borders, and 10 to 20 cattle, said Betty Lieurance, a National Parks Service spokeswoman.
The horses have trampled the surface of archaeological sites, run off native deer and elk and behaved aggressively toward workers and tourists, said Paul Morey, Mesa Verde's wildlife program manager.
"We've had employees and some visitors that have been chased by horses," he said.
Last year, an employee was driving in the park when she rounded a curve and hit a horse, totaling the vehicle, he said.
Park officials have documented about 20 sites that were damaged by horses. Many of the park's lesser-known archaeological sites are easy for the horses to disturb, Morey said.
"Most of the archaeological sites are not as obvious as the Cliff Palace," he said. "They're getting into these sites and trampling them."
Mesa Verde is developing a management plan to deal with the problem. The comment period began Thursday and ends Feb. 28.
Feral horses pose problems throughout the West, but rarely do they settle in areas as sensitive as Mesa Verde.
Mesa Verde is also one of the Four Corners' most popular tourist attractions, hosting 572,000 visitors in 2011.
The horses come from tribal lands to the east, Morey said.
"They're coming from the Ute Mountain Ute tribe," he said. "Since they're unbranded, we can't really force them to come up and get them."
The tribe has conducted roundups in years past, and worked to reduce the wild horse problem. But the tribe is not responsible for Mesa Verde's horses, a Ute Mountain wildlife official said.
"The reality of it is, who could claim ownership of feral horses?" said Jerald Peabody, natural resources director for the tribe. "If they're branded horses, of course the owner would claim them."
Past roundups using local cowboys, ATVs and motorcycles were not cost-effective, Peabody said. A three-year program caught only 278 animals, including horses.
"If you know anything about wild horses, they're strong and fast and they're going to go where they want to go," he said.
Some horse advocates have questioned the use of roundups in the West, saying they stress the horses and are conducted largely to benefit agricultural interests.
Suzanne Roy, director of the American Wild Horse Preservation Campaign in Hillsborough, N.C., favors using birth control, typically delivered by a dart.
"Generally, the National Parks does have experience managing horses and keeping them down to a small level using birth control," she said.
Morey said birth control can be difficult to administer because a wildlife officer must get close enough to an animal to fire a dart, and it must be done every year to be effective.
Mesa Verde is considering several options for dealing with the horses, including birth control and roundups, he said.
"It'll depend on what we get from comment and the public," Morey said. "We'll definitely look at all the options to make it the most humane way of doing it."
Rounding up the horses with a helicopter is a last-resort option, and perhaps not an option at all, Morey said.
Helicopter roundups are controversial because they are thought to cause stress to the horses.
Debbie Coburn of Four Corners Equine Rescue in Flora Vista said she's sympathetic to the park's horse problem. She said a roundup conducted by baiting and trapping the horses in a fenced enclosure is the best option.
Federal and tribal officials have let the problem worsen, Coburn said.
"There has been a lack of management by all parties concerned for years," she said. "There needs to be proactive management. The Parks Service and the Ute tribe need to get their heads together and work on this for the benefit of the horses, for the park and the Ute tribe."