By now, just by the heft of the tiny ball of feathers, Larry Niles can tell if a small shorebird that migrates through Delaware Bay every spring has reached optimum weight.

This year, most didn't.

And that's an ominous sign for the bay's bird life and its status as a renowned bird haven.

The birds, called red knots, depend on the lipid-rich eggs of the bay's horseshoe crabs for refueling, and this year there simply weren't enough crabby morsels for the birds to dine on, says Niles, who has studied the red knot's falling numbers for more than a decade.

But, he says, the decline "is not just about red knots. This is about the bay itself."

The bay's role as a refueling stopover for migrating shorebirds is collapsing, says Niles, chief biologist with the nonprofit Conserve Wildlife Foundation of New Jersey. "The expression of that is the decline of the red knots."

As the final red knots took off last week to complete their epic 10,000-mile migration from the tip of South America to nesting grounds in the Arctic, Niles feared that many would lack the fat reserves to make it. And that the decline of the species would continue.

Computer models from several years ago have predicted the birds could go extinct by 2010.

While conservation efforts have saved birds like the bald eagle and the peregrine falcon, the passenger pigeon and about 10 other bird species, including two species of sparrows and three types of ducks, have disappeared since the days of John James Audubon, who died in 1851.

Once numbering nearly 100,000 on the bay, the red knot population is now about 15,000.

Biologists believe the species is a harbinger for other shorebirds, which are in decline around the globe.

The bay stopover is a shorebird "Serengeti," one of the most important bird congregations in the world, says Eric Stiles of New Jersey Audubon. The spectacle attracts bird-watchers - and their dollars - from far and wide.

Yet shorebird numbers on the bay have dropped from 1.5 million in 1988 to between 300,000 and 400,000 this year.

Within that, species that also depend on crab eggs - ruddy turnstones, sanderlings and semipalmated sandpipers - "are following the same trend line" as the red knot, says Stiles. The only reason they're not in as serious trouble, he says, is that they started with larger numbers.

Niles and others blame the red knot's decline on overharvesting of horseshoe crabs, used as bait for conch and eel, edible delicacies in Asia.

Crabbers contend there are plenty of crabs, which are in no danger of extinction. There just aren't enough crabs for the birds, biologists say.

In March, New Jersey banned the harvest indefinitely, although today the Senate environment committee is expected to consider amendments that critics say would weaken the ban. A regional fisheries commission has enacted limits in Delaware.

Biologists have determined that the knots need a density of about 50,000 pearlescent crab eggs per square meter of beach to feed efficiently.

But this year, a May storm swept the eggs from the beaches. Afterward, the crab spawn, exquisitely timed to the arrival of the birds, slowed.

Counts showed an average density of fewer than 500 eggs per square meter.

In 1997, about 80 percent of the birds leaving the bay had bulked up to a needed weight of about 180 grams, or six ounces.

This year, based on data obtained from birds that were captured in nets, only 15 percent - the lowest proportion biologists have yet seen - gained sufficient weight.

Holding one in his hand, Niles knew instantly: "This guy's not going to the Arctic. If he does, he's going to die, or he's not going to produce."

Contact staff writer Sandy Bauers at 215-854-5147 or sbauers@phillynews.com