In one of the more controversial rites of spring across the National Park System, Yellowstone National Park bison are being hazed on the ground and from the air out of the Montana countryside.

The operations, which at times employ helicopters buzzing as low as 20 feet off the ground along with hazers on horseback or riding ATVs, began this week just west of the park. The hazing is authorized under the Interagency Bison Management Plan that federal, state and local authorities hashed out back in 2000. Under that plan, brokered by the state of Montana, the National Park Service, the U.S. Forest Service, and the U.S. Agriculture Department's Animal and Plant Health Inspection Service, the goal was to come up with a way to prevent Yellowstone bison from spreading brucellosis to cattle beyond the park's borders. (There has, however, been no documented case of such a transmission, although there have been suspected transmissions from elk to livestock.)

A year ago the hazing at times created harsh images of bison -- young and old -- being forced to ford the Madison River as it ran high with snowmelt.

In trying to see the hazing end, the Buffalo Field Campaign churns out a mix of at-times graphic videos and still photographs of the hazing operations and supplements them with press releases critical of both the Montana livestock industry and the federal agencies, including the National Park Service, that developed the hazing plan.

A formidable threat awaits these buffalo. Not the natural predator of bear, wolf, or lion, but the beast of greed that takes human shape: government agents unjustly catering to livestock interests will soon arrive with harmful intentions to force these buffalo - with horse, ATV, truck, helicopter and law enforcement - off of their native habitat in Montana. On Monday, BFC witnessed the season's first harassment operation as Montana Department of Livestock's Shane Grube and Jim Smolzynski, local game warden for Montana Fish, Wildlife & Parks, hazed a group of 18 buffalo off of a section of their migration corridor along the lands south of the Madison River. There is a small private ranch there, but no cattle will arrive until late June. This one private cattle pasture, one landowner, drives the government's shameful mistreatment of America's last wild buffalo on this portion of their habitat.

So far this week the hazing operations involved moving bison off of private ranchland and onto the Gallatin National Forest, according to a representative of the Buffalo Field Campaign. Peter Bogusko on Thursday couldn't cite any instances that could be interpreted as animal cruelty, such as broken legs sustained during the hazing, or agents trying to push the animals across high-running streams or rivers.

But at the Greater Yellowstone Coalition, Mark Pearson, the group's national parks program director, maintained that bison cows giving birth were caught up in the hazing.

"Mother bison are literally in the throes of giving birth while livestock agents swoop in on them by helicopter, ATV, and horseback," said Mr. Pearson. "Most Americans would find this treatment of Yellowstone's iconic bison utterly abhorrent.

"The (Montana) Department of Livestock's indefensible annual show of force runs completely counter to the on-the-ground reality near Yellowstone National Park. A great many people worked hard on voluntary buyouts and other means to keep Yellowstone bison and cattle separate and ensure these are places where there is absolutely no chance for cows to be at risk," he added.

"The Department of Livestock and the Montana Stockgrowers Association should respect those voluntary efforts, along with the private-property rights of landowners who want bison allowed to roam on their property and the desire of the American people to not see this icon of the West harassed. It's well past time to stop this wholly unnecessary charade that results in stress, injury and even death for Yellowstone bison."