Nearly one-quarter of the northern elk herd at Yellowstone National Park is missing, according to the annual winter count, but biologists aren't sure if there's been a stunning decline in the herd or if other factors have skewed the tally.

During an aerial survey in late December biologists counted 4,635 elk, a 24 percent decline from the 6,070 animals counted a year earlier. While the one-year decline seems dramatic, Doug Smith, Yellowstone's wolf project leader, said a number of factors lead him to question "how good this count is."

Heavy snows in November and December could have pushed many elk out of the park, he said, and the aerial tally, which normally has an error rate anywhere from 10 percent to 50 percent, could have been significantly off.

Elk are among the iconic animals that Yellowstone tourists expect to see during vacations to the park. The northern herd -- one of the park's seven elk herds -- is particularly photogenic, with small bands milling around park headquarters at Mammoth Hot Springs and larger numbers readily seen in the rolling grasslands that rim the park road that winds from Mammoth Hot Springs to Cooke City just beyond the park's northeastern entrance.

When Yellowstone officials ceased artificially capping the herd at roughly 3,000-4,000 animals back in 1967, there was a population explosion that saw the northern herd expand to nearly 20,000 animals, Dr. Smith noted Wednesday during a telephone conversation.

That number steadily has been coming down through the last decade or so, in part due to more predators in the park than in past decades. Wolves were returned to Yellowstone beginning in 1995 to restore the park's full complement of predators, and mountain lions also found their way back on their own. Those two predators, along with ever-opportunistic grizzly bears, have helped tamp down the elk numbers, the wildlife biologist said.

Hunting in Montana just beyond the northern range also plays a role in population control. (Come winter, as many as half of the elk in the northern herd head to lower ground; in this case, Montana, according to park officials.)

Dr. Smith can't accept that predation alone has cut nearly a quarter of the northern herd, especially not when the park's wolf population is ebbing due to disease. In the park's northern range just 37 resident wolves were counted last year, according to park officials.

"Is predation a factor? Absolutely, a huge one. But we can't weight it," he said. "The feedback is that it's all wolves, and that's not the case."

Heavy snows in late November and early December "could have changed the elk distribution entirely. And about half the time we do the count in January or February. So we have an unusual snow amount, and the timing is different."

Yet another factor is the hunting season in Montana.

"This unusual snow event pushed a lot of elk out of the park, and they had a really good hunting season, said Dr. Smith.

Another factor that could be in play is climate change. A drier Yellowstone means less forage for elk, which means cows head into the rut with less fat reserves to help them through both winter and their pregnancies. As a result, reproduction can fall. While this past fall and early winter was wet, the trend during the past 15 years has been drought, according to the wildlife biologist.

At the Montana Department of Fish, Wildlife, & Parks, Livingston-based wildlife biologist Karen Loveless agreed the early snow did push elk out of the park and into Montana. But she couldn't say large numbers of Yellowstone elk made the migration.

"We had early heavy snowfall. That occurred toward the end of the general season. In the early hunting season we had light harvest, then the last 10 days, two weeks we did have substantial bull harvest," Ms. Loveless said, adding, though, that "I honestly doubt that the harvest that we had down there could account for the drop in the numbers.”

The Montana biologist also said that during her flights over the hunting districts just north of Yellowstone she didn't notice an inordinate number of elk moving out of the park. If there was a big movement, she had no idea where they might have gone to be out of sight.

“Obviously, it’s deep snow and it's mountainous and there’s not another winter range that I think they would move to," said Ms. Loveless.

“The only possibility is if they continued further north up the Paradise Valley," she said, only to add that, "I've been flying over that and haven’t seen a substantial number of elk up there.”

According to a park news release, there has been about a 70 percent drop in the herd's size since 1995, when there were 16,791 elk counted. That was the same year the wolf recovery program came to Yellowstone.

"Predation by wolves and grizzly bears is cited as the major reason for the decline in elk numbers," the release noted. "Wolves in northern Yellowstone prey primarily on elk. Also, predation on newborn elk calves by grizzly bears may limit the elk population’s ability to recover from these losses."

When asked why Yellowstone officials opted to put out a news release describing a 24 percent decline in the northern herd when the tally was in question, Dr. Smith was quick to answer.

"Transparency. Honesty. Many people think we're lying and trying to do a cover-up to hide the effects of wolves, so the only way to dispel those myths is full disclosure of what you get," he said.

While park officials are debating whether to do another count this winter, Dr. Smith said there's one positive if the decline noted in December proves true.

"If there's a silver lining here, it's a smaller, healthier elk herd," he pointed out.