23 October 2004

(60) Massacre

Four Ravens wipe out 141 grebes in Yellowstone

Terry McEneaney has known for a long time that ravens are efficient predators.
But last May (2004), the Yellowstone National Park ornithologist saw something that really blew his hair back: four ravens systematically killed 141 grebes that were stranded on the icy surface of Yellowstone Lake.
"I was flabbergasted," McEneaney said this week. "I've seen a lot of predation on birds, but that was remarkable."
His account of the spectacle appears in the new issue of Yellowstone Science, a quarterly magazine.
Western-eared grebes don't generally spend much time in the park, but on May 1 hundreds of them were passing overhead, on their way to northern summering grounds.
McEneaney was at the lake, looking for bald eagle nesting sites, when he saw a grebe on the frozen surface.
"I didn't think much of it," he said. The bird had apparently been fooled by shifting cloud shadows into thinking it was landing on open water.
"Then I saw another grebe, and another grebe, and another grebe," he said. "They were all over the place."
Grebes need open water for takeoff, so once they landed, they were stuck there.
"Their legs just spin," he said. "They can't get any traction."

Enter the ravens.

At first, there was only one of the common black croakers 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.
It left the bird where it lay, then moved to another grebe and pecked it to death, too. Grebes weigh about 11 ounces. Ravens are much bigger.
Twenty minutes later, at 11:40 a.m., three more ravens joined in and started killing grebes.
"They started going ballistic," he said.
Then two bald eagles showed up and started eating the grebes the ravens had killed.
McEneaney watched the spectacle for three hours, and, since he's a scientist, he 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.
"When there's an abundance of food, they cache it," McEneaney said of ravens, a bird he has studied for nearly 20 years. "They went back and forth, back and forth."
Eventually, he had to leave. But when he returned at the end of the day, all 141 grebes on the ice that day were dead.
McEneaney said he's watched ravens carry off 13 baby magpies. He's seen them pluck trout from a stream, and once he saw a raven try to peck a mired bison to death, starting with its eyes.
Most people consider ravens scavengers, he said, but they'll kill food when they can.
"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 wasn't safe and park regulations require people to let nature take its course.
"That's what it's all about," he said. "As gruesome as it sounds, it was really interesting to watch. It's etched in my mind."