Today's winter visitors often fall prey to thieves of another sort: sly ravens that have learned to pry into snowmobile storage compartments and unzip backpacks in search of food. Amid such ransacking, ravens also make off with all sorts of other cargo including keys, clothing, maps, sunglasses - even small cameras.

"If you leave anything unattended, you'll probably lose it," said Yellowstone bird biologist Terry McEneaney. "If you leave keys on a snowmobile seat, they'll fly away with it. I've seen them fly away with cameras. If there's some way of getting it in their bill, they'll take it."

Park rangers have now resorted to posting signs in snowmobile parking areas at Old Faithful and other places in the park warning visitors to secure their valuables from the bandits with wings. Just tying something down may not be good enough: ravens can easily untie knots.

"You can't outsmart these ravens," McEneaney said. "They can figure things out as fast as you can."

Raven thievery has always been a fact of life in Yellowstone, but likely increased along with raven populations following the poisoning of predators in the early part of this century that incidentally killed many ravens in and around the park, he said. Their pilfering becomes most pronounced in the winter, when natural food sources - rodents, smaller birds and carrion - are usually harder to find.

"They're relaying on human resources when things get tough," McEneaney said.

Raven populations are growing across the country and probably in Yellowstone, too. More and more human traffic during the winter has offered the roughly 100 to 200 pair of ravens that now dwell in Yellowstone through the winter more opportunities to exercise an intellect that among birds is second only to parrots.

In recent years snowmobile manufacturers have turned to Velcro to close the fabric covers of storage compartments, offering ravens a renewed sense of opportunity they have eagerly exploited by figuring out how to yank open the flaps. One recent day in the parking lot of Old Faithful, a small legion of ravens hopped from one snowmobile to the next, pulling open the flaps as easily as you pop the top of a soft drink can, and hauling out the contents.

The birds discarded items of little interest - Kleenex, for instance - and made off with tastier items like bags of potato chips.

"They first started doing this around Canyon," McEneaney said. "It's this particular ability to get into snowmobiles that has since spread around the park. I don't think it's really one bird learning from another. They do a lot of trial and error stuff and I don't think it takes them long to learn how this works on their own."

Ravens seem to zero in on any unusual texture or imperfection in a snowmobile seat that might suggest a hidden compartment and then they test it for access, he said.

The only real defense against the thievery is keeping food and other precious items secure and keeping heavily used sections of the park as clean as possible, forcing ravens to depend as much as possible on their natural foods instead of human leftovers and garbage, McEneaney said. Capturing and moving ravens is virtually useless because the birds can easily fly back to their original stomping grounds. Even if they do not, other crafty ravens will probably take their place.

Ravens weigh up to four pounds, mate for life and live 20 years or longer - more than enough time to become wise to the ways of humans.

"Inquisitive and quick to learn, corvids (the family that includes ravens, magpies, jays and crows) have some of the most highly developed brains known among birds, and the raven's is the largest in the family," says an article in the January issue of National Geographic, which includes photos of ravens looting snowmobiles in Yellowstone.

When ravens cannot filch food from people, they call for help. Ravens often are the first to spot animals that die during harsh winters and then cry out, attracting coyotes and wolves that can better tear open the carcasses, making the tender insides available to all comers - including, of course, the ravens themselves.

McEneaney calls ravens "raptovengers," he said, because they resemble eagles and other raptors in hunting down rodents and other prey when it's abundant, but are also effective scavengers when opportunity knocks.

A study begun last year by a University of Vermont graduate student aims at documenting interaction between ravens and wolves, a relationship that at times seems almost playful. McEneaney himself has seen ravens play catch by dropping rocks and catching them in mid-air. He once hauled out his tools to free a raven trapped at the bottom of a narrow, 35-foot-deep chimney in the Yellowstone administration building only to find that the bird had evidently climbed its way out.