Right now from the verdant bluff looming over a remote section of shoreline, an observer ought to be able to peer down at swarms of American white pelicans squawking, fluttering and going about the fowl business of breeding.

Yet this year, that perch's vista is instead one of baffling desolation, a plain of baby chick carcasses and hundreds of never-to-hatch eggs simply left behind for the snacking pleasure of hungry coyotes and gulls.

In a quirky and unprecedented natural mystery, the world's largest breeding colony for the birds is eerily vacant. The more than 30,000 pelicans that usually spend the summer procreating at Chase Lake National Wildlife Refuge in central North Dakota returned in their usual droves in April from their winter residence on the Gulf Coast, but then they suddenly dispersed in May after starting an apparently normal breeding season.

Nobody knows for sure why. One biologist, Ron Reynolds of the U.S. Fish and Wildlife Service in Bismarck, N.D., said the birds' diaspora is "as mysterious as crop circles." Theories abound, ranging from possible new predators or climate shifts causing disruptions, but there is little conclusive evidence to make them more than conjecture. Only humans, with minimal access to a region that is 10 miles from the nearest paved road, have largely escaped suspicion.

"Obviously, this is a shock to us," said Chase Lake refuge manager Mick Erickson. "To see something like this happen within a relatively short period of time is both surprising and disheartening. We just don't have any concrete answers as to what kind of event could have caused a mass abandonment like this. . . . We've never heard of any event of this scale ever happening."

Chase Lake, about 60 miles east of Bismarck, was established as a protected area in 1908 by President Theodore Roosevelt to save the dwindling number of pelicans from hunters at a time when the feathers were fashionable in hats and other garments. There were then about 100 pelicans, or 50 breeding pairs; the colony peaked in 2000 at 17,500 pairs on the 4,385-acre site. None of the other 27 breeding colonies in 11 states and four Canadian provinces comes close to those figures, said pelican expert Tommy King of the Department of Agriculture's Animal and Plant Health Inspection Service in Starkville, Miss.

During a normal mating season, the pelicans build nests and lay as many as two eggs in May. Both parents take turns squatting on the eggs to incubate them. Then one parent tends to newborn chicks while the other forages in a 100-mile radius for fish or salamanders for food.

Mating occurs only once a year, so a disruption such as this means a slowdown in reproduction. The birds are not a threatened or endangered species, though, so the incident is not viewed as a serious setback to the species' survival, Reynolds said.

Since May, the Chase Lake birds appear to have dispersed across the Dakotas, Minnesota and southern Canada, according to reports from those regions with sudden influxes in pelicans.

"Normally we may not see any at all this time of year, but the last three or four weeks we've seen 1,000 to several thousand," said Wayne Brininger, manager of the Tamarac National Wildlife Refuge in Detroit Lakes, Minn., about 200 miles east of Chase Lake. "They aren't breeding here. They're just loafing on our lakes and feeding."

Researchers first noticed the abandonment at Chase Lake in late May when they made a routine visit to a peninsula where most of the breeding usually occurs only to find few adult pelicans, some chick carcasses and hundreds of eggs raided by coyotes and gulls. The remaining birds were strikingly nervous around the humans, taking flight when researchers came near instead of walking calmly away as they usually do, said biologist Marsha Sovada of the Northern Prairie Wildlife Research Center in nearby Jamestown, N.D.

That gave rise to the first theory, that coyotes had moved into the area in unusual numbers and scared off the colony. West Nile virus had killed off thousands of chicks in 2003, so researchers wondered whether the coyotes feasted on those remains during the winter and decided the area was a viable hunting ground for them during the summer, too.

But that theory was suspect from the start because there was no littering of adult pelican carcasses to suggest such destruction, and it failed to explain the next discovery of abandonment on one of two islands on the lake where nesting also occurs. Coyotes would not have had access to that area, Sovada said. The same thinking is ruling out a disease outbreak; adult birds would probably have been found dead alongside their young.

By early June, the other island where breeding occurs also was abandoned. King attached global positioning chips to four of the pelicans on the second island in late May before they dispersed, and he found that they fled the area between May 30 and June 2. Their paths -- each in a different direction -- show that something prompted the birds to fan out in haphazard routes, further deepening the mystery.

"This is pretty much an odd, freaky thing," said King, who has been studying the migratory and breeding habits of Chase Lake pelicans for a decade and bemoaned the dispersion as disruptive to his efforts.

Another theory blames a cool, wet spring for stressing the birds and making food more difficult to find. Both May and June have been among the coldest on record for the area, National Weather Service meteorologist Janine Vining said, and the Chase Lake area had three inches more rain in May than usual.

"Abnormally cool weather puts greater physical demands on adult pelicans and may have changed the availability of food source," said Reynolds of the U.S. Fish and Wildlife Service. "That doesn't mean the food isn't out there, but it may be harder to access if small fish are not swimming in shallow waters where the pelicans forage. These birds may need more food and, at the same time, they're balancing the demands of nesting and raising chicks. It could be that amid a little bit of added strain, they chose survival over reproduction."

Experts say they are unlikely to find a conclusive answer -- and they do not know whether the pelicans will return next spring. If not, it would be a significant blow to a region that so treasures its association with Chase Lake that a pelican is painted onto the water tower in Medina, a tiny burg of 500 residents about 10 miles from the refuge.

It could also be bad for business, said Medina innkeepers Janean and David Schmidt. The Schmidts fear they will greet fewer bird-watchers this summer at their Chase Lake Country Inn, a six-room 1916 farmhouse they bought last year after moving from a Minneapolis suburb.

"If they don't come back next year, I would be particularly worried," Janean Schmidt said. "The birding up here goes along with the pelicans. Some of these birds they can see elsewhere, but we're told they come here to see the pelicans."

Pelican eggs lie abandoned on the peninsula of North Dakota's Chase Lake National Wildlife Refuge. Since May, the Chase Lake birds have vacated their usual home, dispersing across the Dakotas, Minnesota and southern Canada.