Vast populations of America's shorebirds risk disaster each spring and fall during continent-spanning migrations that put them at the mercy of man and pollution.

Many of the little sandpipers and plovers that so delight beachgoers with their antics are hemispheric travelers and clock more air miles a year than a jet-setter.They nest in the Arctic tundra in the summer, then set their compass for South America, often making heroic, non-stop flights of 2,000 or more miles at speeds up to 60 mph.

Before attempting such trips they congregate en masse on fertile feeding grounds to gorge themselves with food.

This, scientists say, is when they are most susceptible to a single, catastrophic event that could all but wipe out an entire species.

It could be an oil or chemical spill in a bay or harbor where they gather, pollution in a feeding area, beaches so crowded they cannot properly feed, or man's siphoning of water from a marsh or river basin they have traditionally used.

"When you put all your birds in one basket . . . they are all at risk to a single event, and it totally changes the way we think about what makes a species endangered," said Dr. J.P. "Pete" Myers, senior vice president for science of the National Audubon Society.

The shorebirds use three routes, the eastern, central and western flyways, in their commute between the Canadian Arctic and such wintering spots as Chile, Peru, Argentina, Mexico, Panama and Suriname.

As soon as their young are able to fend for themselves, the older birds leave the tundra in late summer and begin fattening themselves for their hemispheric hop. Some feed up to 20 hours a day and increase their weight by nearly half.

By early the following spring the restless shorebirds have hit the end of their long, elliptical loop in South America and are battling their way northward again, counting on food at strategic spots to fuel their return to the tundra for still another season.

At Delaware Bay they time their arrival with the breeding season of the horseshoe crab, settling down on the beaches just as the crabs come ashore to deposit their eggs by the billions.

Each stab of the bill is rewarded with a meal, and they eat until they have stored enough to continue their journey. Back in the air they sometimes soar to heights of 20,000 feet and beat on relentlessly for 20 or 30 hours until they reach the tundra.

Working against this age-old migration cycle has been the steady disappearance of habitat. Dune buggies and three-wheelers now roar over beaches that were once the domain of birds, and man has filled in many of the marshes in his quest for more and more waterfront.

The result has been that only a relatively few refueling stops remain where the birds can feed uninterrupted and find food in the amounts and concentration they need to sustain them on their marathon flights.

Delaware Bay is one, and San Francisco Bay is another. But Delaware Bay is a busy sea lane for tankers, and thus highly susceptible to oil spills. San Francisco Bay is ringed by cities and industrial complexes.

An estimated 90 percent of San Francisco Bay's habitat has been lost to development, and much of what remains is polluted.

Other key areas include Cheyenne Bottoms, in Kansas; Stillwater National Wildlife Refuge, east of Reno, Nev.; Mono Lake, in California; Grays Harbor, in Washington; Boundry Bay, in western Canada; the Copper River-Berring River delta system in Alaksa; the Bay of Fundy in Nova Scotia, and the Outer Banks of North Carolina.

Another stopping place in previous years was Kesterson National Wildlife Refuge, near Los Banos, Calif., but cannons firing blanks now boom over the waters there to purposely frighten the birds away.

Selenium, which like arsenic is a heavy metal and a poison, found its way into Kesterson's waters when the federal government permitted used irrigation water to be funneled into the refuge.

Birds died by the thousands and the offspring of those that survived were found to have gross deformities.

Stillwater, which has the largest nesting colony of white pelicans in North America, and is a critical migratory stopover for shorebirds, also is troubled by pollution as a result of contaminated irrigation water.

The water was channeled into the refuge in an attempt to keep its marshes flooded, since much of the normal fresh water supply has been diverted for agriculture.

Competition for water is becoming intense in many parts of the west.

The level of Mono Lake is dropping and the lake has become saltier because Los Angeles is siphoning off water.

Cheyennne Bottoms, a huge inland marsh, is but a shadow of its former self because agriculture is claiming so much water the level of the Ogallala Aquifer is dropping by 10 feet a year.

The importance of San Francisco and San Pablo bays was demonstrated last spring when 183 observers from Point Reyes Bird Observator took a two-day census of the area. It revealed 848,470 shorebirds in the bay area, establishing the estuary as one of the four most important in North America.

The count included 550,000 western sandpipers and 32,000 marbled godwits, the latter believed to represent a large fraction of the world's population of this species.

While the San Francisco survey proved impressive, the numbers pale when compared with the Copper River Delta in southeast Alaska, where spring estimates have run as high as 20 million birds.

Brian Harrington, director of the International Shorebird Survey, said should strategic staging migration areas, such as San Francisco Bay, the Copper River Delta, Cheyenne Bottoms, Delaware Bay or Mono Lake "become unavailable to the birds for any of a number of reasons . . . then their populations might plummet rapidly."

In one survey, it was found that 45 percent of all shorebirds that had been counted along the migration route were at Cheyenne Bottoms, including 74 percent of the pectoral sandpipers, 93 percent of the stilt sandpipers, 91 percent of the long billed dowitchers and 91 percent of Wilson's phalarope.

A similar survey at Delaware Bay showed 80 percent of the national total of ruddy turnstones, 79 percent of the red knots, 62 percent of the sanderlings, and 70 percent of the semipalmated sandpipers.

Harrington said "the populations now for most of these species are in good condition," but there are several notable exceptions, including the sanderling, the piping plover and the Eskimo curlew.

Myers said during the past 15 years there has been an 80 percent population decline in the sanderling, the jaunty little bird that darts in and out with each arriving and retreating wave, feeding along the surf line without seeming to get its feet wet.

No one knows exactly why the sanderling is fading, but studies are under way. Myers suspects pesticides or loss of habitat.

Birdwatchers believe that perhaps no more than 1,300 breeding pairs of piping plovers remain, and Myers said "we're lucky if there are 50 Eskimo curlew left."

The Eskimo curlew population was decimated by hunters during the last century and the early 1900s, and they have never been able to recoup.

The shy piping plover has recently been designated as "threatened" on the Atlantic Coast and the Great Plains and "endangered' on the interior Great Lakes.

Myers estimates there are about 200,000 sanderlings and perhaps 300,000 red knots, but he said "we are misled by numbers.

Myers said the decline of the sanderling very well could be a warning to humans, just as the die off of the bald eagle and the peregrine falcon in the 1950s warned man of the dangers of DDT.

"It could be pinpointing a new pesticide, or it may just be pinpointing DDT all over again," said Myers. "We are in the process of studies we hope will help resolve that, but they are in their infancy right now."

Myers noted DDT and a similar pesticide, DDE, are widely used in South America.

"The fact that we have come to grips with the impact of pesticides (in this country) doesn't tell us anything at all about the way our neighbors are handling it," Myers said.

"With sanderlings and with shorebirds in general there's another message that comes through clearly," he said. "These are problems that are not solved by work at just one place. This is a problem that is confronting people all across the hemisphere."