Winters in the seaside marshes. Summers in the prairie ponds that stretch in a sweeping arc from the Dakotas to the Yukon. This has been an invariable rhythm of North American waterfowl for centuries.

But now with each passing year the rhythm grows weaker and weaker. Ducks are in dangerous decline.The bird that provided the inspiration for Daffy and Donald, not to mention Beatrix Potter's Jemima Puddle-Duck, is in trouble.

Farm tractors, beachfront condominiums, raccoons, bad weather, rapacious hunters and even the wanton sexual practices of mallards have conspired to seriously depress the populations of most species of duck.

Today there are an estimated 35 million ducks breeding in North America, 25 percent fewer than in 1957.

The decline has been so relentless and the causes so varied that many wildlife biologists are concerned that the trend cannot be stemmed - that ultimately some of the 29 species of ducks could disappear.

Against this rather bleak picture, the tidal marshes of New Jersey - with their watery channels and maze of reeds - stand out as a striking duck haven.

Here, wood ducks and mallards have been increasing, and the black-duck population has remained steady, according to state wildlife surveys.

But the numbers are deceptive, because while they look good in New Jersey, they are horrible in the rest of the Atlantic Flyway - which stretches from Maine to Florida. For example, in the last six years alone, black ducks have declined 18 percent.

"New Jersey is right on the edge of a major freeze line. . . . With the combination of protecting our salt marshes and . . . the more moderate weather, New Jersey is a good place for ducks," explained Lee Widjeskog, a wildlife biologist for the New Jersey Division of Fish, Game and Wildlife.

He added that as the total number of ducks dwindles, there is also less population pressure and competition to force ducks to winter farther south. As a result, states such as North Carolina and Maryland have seen big drops in species such as the black duck, Widjeskog said.

This spring, the U.S. Fish and Wildlife Service proposed acquiring 14,700 acres of swamp and marsh in Cape May County, N.J., because of the crucial role these areas play in the wintering of black ducks and snow geese, as well as the annual migration of songbirds.

However, waterfowl biologists warn that even with the state's preservation efforts, New Jersey's ducks may be living on borrowed time. "In some regions of the country we have more wetlands than we have waterfowl to fill them," said Jerry Serie, a Fish and Wildlife Service biologist.

In an effort to combat this continental problem, the United States and Canada two years ago drafted a comprehensive North American Water Fowl Management Plan.

The 15-year plan, which is strongly supported by hunters and state wildlife officials, aims to protect nearly 5 million acres of wetlands and tighten hunting controls. Its goal is to restore the duck breeding population to 62 million by the year 2000.

Some waterfowl biologists remain skeptical about the duck's prospects. "I am not very optimistic that we will ever get back to a level which is considered great," Widjeskog said. "There are an awful lot of economic demands out there . . . and the birds are flying right into them."

But just about all the biologists agree that the key to the survival of the ducks lies in the prairie's pothole wetlands where the ducks breed.

As the glaciers of the last ice age retreated back toward the North Pole, they gouged holes, depressions and lakes across 1,000 miles of the continent's central plains.

Each year the spring rains would turn these depressions into ponds and wetlands that would become home to millions of ducks during their summer mating. The majority of North America's ducks are hatched in these potholes.

But in the last 20 years, the competition between duck and farmer for the prairies has grown more intense, and the farmer has been winning. "Farmers are better equipped," said Widjeskog, who has spent several summers doing research in the pothole region.

"With these new four-wheel-drive tractors and huge plows, they are hitting areas they never hit before," he said.

The situation has been compounded by weather and government. Between 1980 and 1985 there was an unusually severe drought that dried up many of the pothole wetlands.

That made it easier for farmers to plow over the land, and since government subsidy policies - in both the United States and Canada - gave farmers an incentive for adding on that acreage, that's just what they did.

In North and South Dakota alone, the pothole wetlands acreage is believed to have dropped from 7 million acres to 3 million acres, according to one Fish and Wildlife Service estimate.

The dwindling of pothole habitat has forced more ducks into the remaining wetlands, and that has led to a host of other problems. As the ducks have been forced to congregate in the remaining wetlands, they have become easier prey to foxes, raccoons and skunks, according to Widjeskog.

And they are also more vulnerable to disease. "There are a tremendous number of deaths each year due to avian botulism, avian cholera and duck viral enteritis," biologist Serie said.

If all this were not enough, there is the problem of the Lothario of the duck world - the mallard. "Mallards will breed with any duck that stands still long enough. . . . They are extremely sexually active," Widjeskog said.

The offspring of these liaisons are hybrids, like mules, and cannot reproduce. So a lot of mating, nesting and food-gathering energy leads to a biological dead-end.

The result of all these forces - loss of habitat, predators, disease - is lower reproduction rates. "It appears that birds are no longer as productive," Serie said.

Finally, there are the hunters who blast millions of ducks from the sky as the birds fly south each year. The role hunters play has been a controversial issue.

Wildlife officials have drastically reduced bag limits for duck hunters. In the last few years, the duck "harvest" has been cut from 14 million to 9 million birds.

Widjeskog predicted that "eventually the seasons will be restricted even more or closed entirely to protect the birds."

Fish and Wildlife Service biologists, such as Sparrowe and Serie, stress that ducks are not yet endangered species and that they have shown the ability to rebound. "In the 1970s," Serie said, "the breeding population jumped up."

Then came the drought and the heavy plowing on the prairies. "If we preserve habitat, manage the populations carefully and control hunting," Serie said, "I think the ducks can make a comeback."