Leggy, spindle-beaked and fond of wading in the ripples along ocean beaches, sandpipers don't look like a cold weather bird.
But one subspecies -- and only one -- spends winters on the frozen mudflats and ice floes of upper Cook Inlet, where low temperatures from December to February can average 10 degrees or less.
"It's far and away the coldest non-breeding site in the world regularly used by shorebirds in the winter," said Dan Ruthrauff, a biologist with U.S. Geological Survey Alaska Science Center.
Ruthrauff, lead author in a series of recent scientific articles about the bird and its ability to take a chill, said he's seen individuals in freezing temperatures with ice on their legs and feathers or encasing their feet.
"For an ordinary bird, that would be lethal. But those guys were foraging normally and seemed fine," he said.
The subspecies in question is the Pribilof Island rock sandpiper, a relative of curlews and snipes described by ornithologists as "robin-sized." As the name suggests, it breeds in the Pribilofs and on uninhabited St. Matthew and Hall islands, about 750 miles west of Anchorage, far out in the Bering Sea.
Their exact migration pattern isn't known, Ruthrauff said, but "they start trickling into Cook Inlet in mid- to late-October," when other shorebirds are heading south. They stick around the part of the Inlet roughly north of Kenai until late March or early April, when the fair weather shorebirds return.
Biologists have found an average of 8,000 in the upper Inlet in a given year, but in some years the number can hit 20,000, essentially the entire population of the subspecies as estimated from surveys in their Bering Sea nesting grounds.
In other years, they nearly vanish. Why "is one of our burning questions," Ruthrauff said.
Instead of flocks of thousands, groups of a few hundred may be spotted in locations closer to the Gulf of Alaska, Homer, Cordova, Kodiak, sites where shorebirds abound year-round.
Though in the Pribilof Island rock sandpiper's winter hot spot, Anchorage sees few of the birds. Sightings in the annual Christmas Audubon Society bird count have swung from 0 to 300 back to 0 during the past 10 years.
"They sometimes occur in small groups off the bluff near the Jodhpur road motocross park," Ruthrauff said. "But the mudflats around Anchorage typically get too clogged with stranded sea ice during the winter. The (west) side of the Inlet has strong currents that tend to keep the mudflats more free of icebergs, thus ensuring ice-free mudflats upon which they can forage."
Their food is the Baltic tellin, a thumbnail-sized clam that teems in the Inlet mud, as many as 2,000 in a square meter.
"These birds must consume the clams whole and crush them in their muscular gizzard, a lot of work," Ruthrauff said. "They're voracious little clam eaters."
In trying to determine why this group of sandpipers winters here and others move on, the scientists initially speculated that they must have "either unique metabolic or foraging adaptations." But in the study published last month in the journal Auk, researchers found that the bird's metabolism wasn't different from other rock sandpipers.
"But they do have an inherent ability to really elevate their metabolic rates in response to cold," Ruthrauff said.
Many shorebirds show elevated metabolic rates when migrating, he said. "But few species," if any, "maintain such high metabolic rates all winter long in lieu of migrating to more benign environments. So it's an unusual strategy."
They seem to be better insulated against the cold than other sandpipers, with heavier plumage and a thicker layer of fat that Ruthrauff likened to blubber on a whale or seal. The fat not only insulates them but fuels their elevated metabolism, cranking up the transfer of clam meat to calories like little round fluffy Yukon stoves.
When not feeding or flying, they conserve energy by scrunching their heads into their torsos, tucking up their legs and sitting down on their feet. "They pretty much try to approximate a ball to minimize heat loss," Ruthrauff said.
Ruthrauff and fellow researchers are continuing to investigate how the bird thrives even when up to its skinny knees in ice. At the moment, exceptional foraging skills seem one likely answer, he said. "I'm not done with the analyses, but the Pribilof rock sandpiper seems to be especially adept at finding, manipulating, swallowing and processing the Macoma clams."
Despite flocking within a few miles of Alaska's largest city, the sandpiper's presence in Cook Inlet remained unknown to scientists until recently.
"My just-retired supervisor Bob Gill stumbled upon this in 1997 during a mid-winter aerial survey," Ruthrauff said. "Given the miserable cold and ice, no one even bothered to look for shorebirds here in winter, because it's 'obviously' too cold for shorebirds.
"How wrong we were."
"If you pop across the Inlet to Beluga, they are quite common," he said. "But there's really no one over there to see them in the winter, so they remained 'undiscovered' for quite some time."
"No one" is hyperbole, he acknowledged. People have lived on the west side of the Inlet across from modern Anchorage for hundreds or thousands of years and still do in the Native village of Tyonek.
"Not surprisingly, subsequent discussions with folks in Tyonek demonstrated that some people knew they were there all along," Ruthrauff noted. "Just not us nosy biologists."
Now that biologists do know about them, they can make some educated assertions. For instance, the fact that such a large number of one subspecies concentrates in a relatively small area "potentially makes them pretty vulnerable," Ruthrauff said.
A convergence of several factors mean that upper Cook Inlet is an important and one-of-a-kind resource for the birds, he said. "It makes for a pretty unique wildlife occurrence."
ACADEMIC JOURNAL PAPERS on Cook Inlet's Pribilof Island rock sandpipers written by Daniel Ruthrauff and others include:
• "Small population size of the Pribilof rock sandpiper confirmed through distance-sampling surveys in Alaska," in Condor, Aug. 2012
• "Identical metabolic rate and thermal conductance in rock sandpiper subspeceis with contrasting nonbreeding life histories," in Auk, Jan. 2013
• "Coping with the cold: an ecological context for the abundance and distribution of rock sandpipers during winter in upper Cook Inlet, Alaska," accepted for publication in Arctic in Sept. 2013.
Ruthrauff is currently working on a doctoral thesis about the sandpipers and their unusual adaptation.
Reach Mike Dunham at firstname.lastname@example.org  or 257-4332.
By MIKE DUNHAM