AMONG NATURE'S MIRACLES, the migration of birds ranks high. These tiny creatures, impelled by instinct and faith, perform almost unbelievable feats of physical strength and navigational skill.

The migration is on, and throughout April and May residents of the Wasatch Front can see some of close to 100 species of shorebirds and other water-dependent fowl that annually stop at the Great Salt Lake, going north and returning south, or in many cases remaining here to nest and summer.

Since the coming of the first settlers, and probably long before that, the Great Salt Lake has been a magnificent ecosystem supporting enormous numbers of birds - waterfowl, herons, egrets, gulls, terns, pelicans, ibises, raptors and songbirds.

In recognition of the lake's utmost importance to wetland-dependent birds, the Western Hemisphere Shorebird Reserve Network (WHSRN) has identified the Great Salt Lake as a Hemispheric Site, a critical link in the chain of primary migratory, breeding and wintering sites along the great shorebird flyways that extend from the Arctic to the southern tip of South America.

As with any other chain, the network is only as strong as its weakest link. By becoming a part of the network, a site gains international recognition and support for local conservation efforts and wetland management. To qualify, a site must entertain in excess of 250,000 birds a year, or more than 30 percent of a species' flyway population.

The Great Salt Lake is host to millions of birds in a season, but Don Paul, regional nongame manager for the State Division of Wildlife Resources, explained that the lake qualifies as a hemispheric site on the basis of the numbers of Wilson's phalaropes here during July and August, en route to South America.

"I've seen flocks of 500,000 to a million phalaropes," he said. "They tend to filter north in smaller flights in the spring, because the food is not so abundant now as later. Many breed here, others go farther north in the U.S. or to southwestern Canada."

The WHSRN has twinned the Great Salt Lake as a sister reserve with Laguna del Mar Chiquita, a salt lake in the Cordoba province of Argentina, where more than 500,000 Wilson's phalaropes spend the winter.

"Shore birds are among the longest distance migrants," said Margy Halpin, an urban wildlife biologist for the Utah Division of Wildlife Resources. "More than 20 million shore birds migrate through the U.S. to breeding grounds as far north as the Artic every year, then back to wintering sites in South America, traversing as much as 15,000 miles in this annual circuit!

"Though not quite so large a site, the Great Salt Lake is typical of several key staging areas along their route where these little travelers congregate in enormous numbers - unique places that they depend upon because of their super-abundant food resources. There the birds replenish energy reserves, then continue their migration, often non-stop to their final destination."

Several examples of such strategically situated locations, essential to bird migration, are the Copper River Delta in northern Alaska, and Gray's Harbor, Wash., along the Pacific flyway, and at the top, Canada's Bay of Fundy. The Cheyenne Bottoms in Kansas is integral to the central flyway; and Delaware Bay in New Jersey nourishes untold numbers along the Atlantic flyway.

So vital are these staging sites that at times large percentages of entire populations are at a single place. For example, 80 percent of the U.S. East Coast red knots depend upon horseshoe crab eggs in the Delaware Bay; and nearly 100 percent of western sandpipers and dunlins use the Copper River Delta.

Utah biologists have learned much from Joseph Jehl's research at Mono Lake, a salt lake in California also frequented by Wilson's phalarope. The phalarope's annual cycle sheds light on how the Great Salt Lake fits into the migratory scheme of things.

These pretty birds about the size of a robin, blue-gray with rust and black breeding plumage, like the Great Salt Lake because of its plenteous brine shrimp and brine flies - humble waterlife that also play a key role in servicing a myriad of other birds in their southern migration. (In fresh water bodies, fish and other predators eat such tiny invertebrates. But since predators are non-existent in the Great Salt Lake, brine shrimp and brine flies multiply in copious quantities.)

The female phalarope has beaten the system. After she lays her eggs she does not guard the nest; the male takes over incubation, and she heads south. Thus south-bound female flocks begin appearing at the Great Salt Lake about mid-June. Baby phalaropes hatch precocial, so that they quickly become independent; so by July large flocks of males follow, with some juveniles, then many juveniles.

North-bound adults in spring wear their mating plumage, so when they stage here in late summer they must go through the energy-consuming process of moulting and replacing flight feathers. Eating the brine shrimp and flies, they nonetheless gain about an additional third of their body weight to sustain them on their long flight south.

No bird can stand much salt ingestation, and phalaropes are salt-resistant. "They forage by swirling in the water in such a way that they don't take on the salt," said Paul. "They really prefer fresh water, they must go to it every day."

Indications are that when the phalaropes leave here, they fly several thousand miles over the Pacific to the vicinity of Laguna del Mar Chiquita - like many other migrants, without touching down before their destination.

Certain other salt-resistant species also use the lake for staging, notably eared grebes, which fly farther north to breed, but have been seen here in flocks estimated at hundreds of thousands to a million, mostly southbound from October to early December. They eat only the shrimp, not the flies.

Shore birds' passage through Utah is more spectacular in late summer and fall, when heading south, than in the spring, when they filter north in smaller, more random flocks. But besides the familiar gulls, sandpipers and killdeer, bird watchers will find a surprisingly vast array that use other lakeside habitats - the open salt water areas, shallow waters, salt marshes, mudflats and salt flats.

Among those known to breed here are the showy long-legged American avocet and the black-necked stilt, which prefer river delta areas where a little fresh water is coming in, said Paul. Other Utah breeders are the snowy plover, willet, and long-billed curlew. Just passing through are the black-bellied plover, red knot, greater and lesser yellowlegs, and marbled godwit.

Has the high water and re-composition of the lake affected bird migration through Utah? To an extent, said Paul, but the lake is still a good habitat for foraging birds.

"We have lost a lot of brine shrimp population in the south arm of the lake since the water has thinned out as low as 6 percent salt," he explained. "On the other hand, in the past the shrimp couldn't stand the high saline content of the north arm, which went as high as 26 percent. Since the north arm is now diluted to around 17-18 percent, the shrimp have shifted up there, and the birds have shifted accordingly."

Most of the birds seen at the Great Salt Lake are migratory, and many nest here. This includes the waterfowl - ducks, geese and swans. Fish-eating birds (such as great blue herons, black-crowned night herons, cormorants, snowy egrets and cattle egrets) that have customarily inhabited the marshes in great numbers, now forage partly in other areas, because the high lake level has decreased their accustomed food supply.

The marshes also support many other species that summer here, including the white-faced ibis and black and Forster's terns. Here are also large colonies of California and Franklin's gulls, that "earn their living by predation," said Paul. "They are the ones you see following the plow, or in the city begging for handouts. Sometimes they do sit on the lake and make a meal of brine shrimp."

Paul has been excited to see large flocks of bald eagles on the wetlands east of the lake. "They winter in Utah, and I counted 110 feeding on dead carp one day," he said. "They summer just 50 miles south of the Artic Circle, they like Canada's Great Slave Lake."