BOZEMAN — A Yellowstone National Park ornithologist who saw four ravens systematically kill 141 grebes describes the spectacle in a recent issue of Yellowstone Science, a quarterly magazine.

Terry McEneaney had known for a long time that ravens are efficient predators. But he was astonished last May when he saw four of them attack so many grebes on icy Yellowstone Lake.

Western-eared grebes generally do not spend much time in Yellowstone, but on May 1 hundreds passed overhead, en route north for the summer.

McEneaney was at the lake, looking for bald eagle nesting sites, when he saw a grebe on the frozen water. Shifting cloud shadows apparently had fooled the bird into thinking it was landing on open water.

“Then I saw another grebe, and another grebe, and another grebe,” McEneaney said. “They were all over the place.”

Grebes need open water for takeoff, so once they landed, they were stuck there.

Enter the ravens.

At first, only one was on the ice. It “flew out to one of the stranded grebes and pulverized it with its long beak until it was dead,” McEneaney wrote in the magazine.

The raven left the grebe where it lay, then moved to another and pecked it to death, as well. At about 11 ounces, grebes are much smaller than ravens.

Twenty minutes later, three more ravens joined in and started killing grebes. Then two bald eagles showed up and began eating the grebes the ravens had killed.

McEneaney watched for three hours and kept track of the action.

After the ravens had killed 92 birds, they began dismantling the carcasses and flying to the shore, where they cached the grebe meat in the snow, said McEneaney, who has studied ravens for nearly 20 years.

“When there’s an abundance of food, they cache it,” he said. “They went back and forth, back and forth.”

He had to leave, but when he returned at day’s end, all 141 grebes were dead.

McEneaney said he has watched ravens carry off 13 baby magpies, pluck trout from a stream and even try to peck a mired bison to death, starting with its eyes. What he saw at Yellowstone Lake in May topped it all.

“I’ve never seen anything of this magnitude,” he said. “I didn’t believe they could do that much predation in one day.”

McEneaney said he never considered trying to intervene. The ice was not safe, and park regulations require Mother Nature be allowed to take its course.

“That’s what it’s all about,” McEneaney said. “As gruesome as it sounds, it was really interesting to watch. It’s etched in my mind.”