BILLINGS, Mont. - There are now at least 1,300 wolves prowling Montana, Idaho and Wyoming, far more than anyone imagined when the species was reintroduced in the Northern Rockies 12 years ago.

The wolf population has, on average, grown by about 26 percent a year for the past decade. The latest estimates, which summarize counts completed at the end of 2006, show they aren't slowing down.

"I keep thinking we're at the top end of the bubble," said Ed Bangs, wolf recovery coordinator for the U.S. Fish and Wildlife Service. "I can't see that there's room for any more, but we'll see."

As the wolf population has grown, so have the reports of cattle, sheep and other livestock being killed. In response, wildlife officials last year killed a record number of wolves after livestock attacks.

It's no surprise that wolves are thriving following reintroduction in 1995 and 1996 in Yellowstone National Park and in central Idaho. Wolves are skilled predators, fast breeders and able to live in different environments.

Over the past decade, they've found plenty to eat and places to settle down.

"It's just really good habitat in the West," Bangs said.

There are at least 316 wolves in Montana, 311 in Wyoming and 673 in Idaho, according to the 2006 federal report.

Bangs said he's surprised that the wolf population continues to grow so steadily. Eventually it will level off, he said, and will likely drop to lower numbers once state agencies take over management and are able to use hunting as a tool to control the population.

The fastest-growing area for wolves last year was in Wyoming outside Yellowstone National Park. The number of wolves jumped by 31 percent, going from 134 in 2005 to 175 in 2006.

With that increase, 123 cattle were reportedly killed by wolves, more than has ever been recorded in Wyoming since the reintroduction. In response, 44 wolves were killed, which is also a record for that time period.

In Montana, the number of wolves grew by 19 percent, nearly all of that in the northwest part of the state, said Carolyn Sime, leader of the wolf program for Montana Fish, Wildlife and Parks.

The wolves seem to be squeezing into areas that generally have been occupied by other packs.

"We're not necessarily expanding distribution," Sime said. "We're kind of filling in a bit."

Several packs came and went last year, but the net result was 14 new packs in Montana, including several established by young males that left other packs in the state.

Two packs took heavy hits during the year after preying on livestock. Fifteen wolves were killed from the Sleeping Child pack in southwest Montana, and 11 were taken out of the Spotted Dog pack outside Avon.

Overall, at least 32 cows and four sheep were killed by wolves, according to the annual report, and 53 wolves were removed.

Human activities - legal and illegal - are the leading cause of death for Montana's wolves, though that's not the case in Yellowstone.

There, the population grew by about 15 percent last year, from 118 to 136. That growth comes after a decline in 2005 attributed to a canine disease that wiped out scores of pups.

But for those that survived into 2006, they were most likely to die at the jaws of other wolves.

Social strife, especially on the Northern Range where wolf packs are densely clustered, and competition for prey meant more territorial skirmishes that can be deadly.

The number of elk, which are wolves' primary winter prey, has declined 50 percent in the area since 1995. A decreasing prey base and increasing wolf density is likely to mean a decline in wolf numbers over the next several years, biologists said.

Ultimately, the survival of wolves in the Northern Rockies won't be determined by conflicts with other wolves, Bangs said.

"People will decide how many wolves there are and where they're going to live," Bangs said.

The Fish and Wildlife Service said the wolf population has, for seven years, met basic recovery goals of 30 breeding pairs distributed across Montana, Wyoming and Idaho.

The agency has recommended removing wolves from the endangered species list. But the process has become mired in a conflict between the federal government and the state of Wyoming.

The latest proposal is to delist wolves in Montana, Idaho and all of Wyoming except for the northwest corner.

If the proposal goes through, Montana and Idaho would take over full management of wolves in those states and would be allowed to use hunting as a way to manage their numbers. In all but the northwest corner of Wyoming - where they'd still be managed by the federal government - wolves would be treated as predators and subject to unregulated killing.

As part of the delisting proposal, all three states would have to provide detailed public reports for at least five years on how wolves are being managed.

Public comments on the delisting are being accepted until April 9.