This is the fourth in a series of articles — produced by the Bay Journal and Chesapeake Quarterly, the magazine of Maryland Sea Grant — that explore the impacts of, and policies related to, sea level rise around the Bay.

When it comes to endurance athletes, few can match the performance of the rufa red knot. The 5-ounce shorebird may fly 19,000 miles annually from its wintering grounds at the southern tip of South America to its summer breeding grounds in the Canadian Arctic and back again.

The red knot often flies nonstop for thousands of miles before landing on narrow sandy beaches where it forages for mussels and clams or, in Delaware Bay, feasts on recently buried horseshoe crab eggs, sometimes doubling its weight in a matter of days.

That migration has gone on for thousands of years, but its future is uncertain. Red knot populations have plummeted 75 percent since the 1980s, leading the U.S. Fish and Wildlife Service in December to list it as “threatened” under the Endangered Species Act. That designation means it could become endangered — threatened with extinction — within the foreseeable future.

The knots’ long migration puts them at various risks all along their route. The birds are threatened by coastal development; shoreline stabilization, that reduces beach area; invasive vegetation; aquaculture; and other activities that take place along the coasts. In some cases, the introduction of peregrine falcons may be taking a toll on migrating birds. During migration, they are also hunted at some stopover sites, including some Caribbean Islands.

But the red knots’ biggest challenge, according to the USFWS, is climate change. Warming temperatures will affect breeding and wintering grounds, along with some food supplies, while rising sea levels in coastal areas — including the mid-Atlantic — will squeeze beach habitats during migration.

It is the first bird to gain Endangered Species Act protection primarily because of climate change.

The birds’ migration is so strenuous that their leg muscles, gizzard, stomach, intestines and liver all decrease in size in preparation for migration, while their pectoral muscles and heart increase in size. Because it takes days for the birds’ digestive system to regenerate, foods consumed during stopovers need to be calorie-rich and easily digested.

The birds do the trip repeatedly: One bird, banded in Argentina in 1995 was nicknamed Moonbird because it logged enough miles to fly to the moon and at least halfway back during 21 years of migration.

But the strenuous journey takes a toll, and human activities all along the way add to the difficulties. For the largest group of migrating birds, the ideal stopover historically was Delaware Bay, where they gorged on the eggs of horseshoe crabs, which are laid on the beach just as the red knots arrive during spring migration. But the overharvesting of horseshoe crabs greatly diminished that food supply, and the number of red knots stopping at Delaware Bay fell from 60,000 annually in the early 1980s to fewer than 20,000 in recent years, according to the USFWS.

The USFWS credited the Atlantic States Marine Fisheries Commission for recent actions aimed at curbing horseshoe crab harvests, though it said it was difficult to say whether those actions would lead to a rebound in red knots. And, a number of environmental groups that have long sought Endangered Species Act protection for the birds contend that the ASMFC’s actions still fall short of what’s needed to protect the bird.

“Now that they’ve been listed as threatened, it’s time to make serious changes to horseshoe crab management and put a halt to their decline,” said Jason Rylander, senior attorney for Defenders of Wildlife. “We’ve waited far too long for this decision, and red knots are paying the price.”

Delaware Bay is not the only migration stopover, nor the only one where red knots face a challenge. Virginia’s barrier islands are the second most important stopping point in the region, hosting about a third of the migrating red knot population. While the number of red knots at Delaware Bay declined sharply over the last 30 years, the numbers using Virginia’s barrier islands had remained steady — until recently.

Results of an annual red knot barrier island survey conducted by the College of William and Mary Center for Conservation Biology and The Nature Conservancy is showing a decline along Virginia’s coast as well. Bryan Watts, director of the Center for Conservation Biology, said that could reflect the rufa subspecies’ broader population decline, but it’s also possible that the barrier islands are facing their own food problem.

The islands were never an important area for horseshoe crabs, but as the islands migrate toward land, marshes behind the islands retreat, exposing a rich layer of peat during low tides which provide an abundance of mussels and clams for the birds. The red knots’ preferred food in that area is the blue mussel, but Watts said that species has undergone a dramatic decline because of climate change. It is extremely sensitive to warm temperatures, and the blue mussel’s range is retreating northward at a rate of 7.5 kilometers per year. Virginia is now at the southern edge of its range.

“If that continues, it is like pulling the rug out from under the knots,” Watts said. “They will get here, but the blue mussels won’t be here to feed on anymore.”

In addition to altering the available food supplies along the migration route, the changing temperatures are also affecting the northern breeding grounds, according to the USFWS. The breeding grounds are getting more dry, and their vegetation is changing. Some also believe that predation is increasing.

Changing temperatures also threaten to create “timing mismatches,” in the food supply where the arrival of food sources — such as the deposit of horseshoe crab eggs — no longer coincides with the arrival of migrating birds.

The potential stopover habitat for red knots once ranged from North Carolina to New Jersey, but beaches are being lost to coastal development. “The places that they had available to them for foraging 50 to 100 years ago were much greater,” Watts said. “Now they are being squeezed into smaller and smaller places, and they are having to do the best they can in what’s left, basically.”

The Virginia barrier islands are unique because they are mostly owned by The Nature Conservancy, USFWS and state of Virginia. “The barriers are only 2 percent developed,” Watts said. “If you go up to the Jersey shore, it is 45 percent.”

If that stretch of good, but contracted, habitat suffers from a food shortage because of the decline of blue mussels, it could cause problems similar to what the birds encountered in Delaware Bay with the horseshoe crab. “It’s a concern that we have only recently identified, but it is looming,” Watts said.

Sea level rise from climate change may further constrict the beach habitats available for red knots, though its impacts will be felt over a longer period of time. Data in the USFWS listing decision showed the mid-Atlantic region suffering from some of the greatest rates of sea level rise in the red knot’s range.

Beaches may be inundated faster than they can regenerate as they migrate inland. Barrier islands are also at risk. They naturally migrate toward the mainland, but faced with the combination of more rapid sea level rise with more frequent severe storms, the islands “may disintegrate rather than migrate, representing a net loss of red knot habitat,” the USFWS said.

Another concern, according to the service, is the rapid increase in the hardening of shorelines that typically accompanies development. Constructed bulkheads aimed at protecting land from being eroded by rising water levels also lead to a loss of beach habitats. About 40 percent of the coastline within the red knot’s range is already developed, and much of that is already stabilized through bulkheads, levees, breakwaters or other structures that reduce or eliminate beach habitats, the service said. “The quantity of red knot habitat would be markedly decreased by a proliferation of hard structures,” according to the USFWS.

While red knots, like most shorebirds, are typically associated with the Atlantic Coast, some also venture into the Chesapeake where they use sandy habitats, but they have never been surveyed or studied as much as those along the coast, Watts said.

“They do use the Bay even though it is not flatly recognized as supporting red knots,” he said. “They come through here every year. Obviously it is not at the scale of Delaware Bay.”

Watts considers the mid-Atlantic to be a “neighborhood of staging areas” where red knots stop and refuel before continuing north, but where Delaware Bay has been dominant. But the Chesapeake could become more important to the birds if populations in Delaware do not recover, and those on Virginia’s barrier islands continue to decline.

The birds are capable of sampling food supplies in different places, and making adjustments in where they go, Watts said. “That is how they survived for thousands of years.” It could result, he said, in neighborhoods like the Bay becoming more important over time — if its beach habitats keep pace with sea level rise.