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CORVALLIS - Wildlife biologists at Oregon State University are finally shedding some light on the biology of the harlequin duck - one of the most elusive waterfowl in Oregon.

Armed with chest waders, binoculars, safety ropes and fine "mist" nets, OSU wildlife professor Bob Jarvis and graduate student Howard Bruner have spent three field seasons walking up many miles of rushing Oregon Cascade mountain streams, searching for breeding and nesting harlequins.

Until the OSU study, very few harlequin duck nests had ever been found in Oregon, except a few discovered incidentally, explained Jarvis.

"There's hardly anything known about them," Jarvis said.

Harlequin ducks spend most of the year on rocky ocean coasts, feeding on mussels, chitons, limpets and other marine invertebrates. Each spring, these small diving ducks migrate inland to mountain streams to mate and rear young.

The scientists hoped to find out what types of habitat harlequin ducks require for nesting and feeding, how successful they are at fledging young, how readily they find new sites if their previous year's nest is destroyed, and whether nesting sites are passed on from generation to generation.

Once they encountered harlequins, Jarvis and Bruner stretched almost invisibly-fine mist nets across the stream just above the water. They herded the ducks and flushed them into the net. Then they fit each bird with identifying leg bands and tiny radio transmitters that were glued and tied to the tail feathers of the females.

The birds could be identified and located from a distance periodically throughout the spring and summer. Young were subsequently banded for future study with colored leg bands.

"It's tricky business," said Bruner. "Sometimes we go for days without seeing a duck. And when you do find them, it's not that easy, because usually in April or May, when they are getting ready to nest, the creeks are swollen from spring snow melt."

Female harlequin ducks lay an average of five eggs per clutch, Jarvis said. The eggs usually hatch in about four weeks and the young are able to swim and feed themselves within a day.

"They've been known to go through culverts, down cataracts, down talus slopes or 50-foot high cliffs when they are only a day old," he said. "They are light and fluffy and they bounce."

Young ducks stay with their mother for much of the summer and she may accompany them on their initial move to the coast. Young fledge about 55 days after hatching.

From their studies Jarvis and Bruner also learned: -Harlequin ducks are extremely faithful about nesting in the same site from year to year. In almost half the nests they studied, there was evidence of use the previous year, such as old eggs shells.

-The ducks feed on aquatic invertebrates, especially caddis fly larvae, while on the steams.

-Female harlequins don't breed until they are three years old. They may pair up earlier, but have not been observed raising young until their third year. Young non-breeding male and female ducks may form pairs hanging around on streams during the spring. Some of the adult pairs remain together for several years.

-Male harlequin ducks come inland only to breed. They return to the coast after females begin incubation, which probably precludes renesting by unsuccessful females.

-Predation is the most common cause of nest failure. Biologists have yet to determine the culprits, said Jarvis, but otters, minks, bobcats, raccoons, skunks, ravens and coyotes are all suspect.

-Most harlequins seem to nest upslope of a stream, often several hundred meters away from the water. They seem to prefer steep slopes with thick plant cover, a clear flyway to the water and streams with boulders and logs. They nest on the ground, on stumps, in cracks or on rocky slopes, and line their nest with a thick layer of down.

Jarvis plans on continuing to band and study harlequins to learn more about how long they live, whether young return to their home stream or disperse to other streams. The study is funded by the U.S. Forest Service, Bureau of Land Management, Oregon Department of Fish and Wildlife, the U.S. Fish and Wildlife Service and Delta Waterfowl and Wetland Research Station.