The rufa red knot (Calidris canutus rufa) is one of the longest distance migrants in the animal kingdom. On a wingspan of 20 inches, the bird flies more than 18,000 miles each year between breeding grounds and wintering grounds.

Nearly 9 inches long — about the size of a robin — the rufa red knot winters at the tip of South America in Tierra del Fuego, in northern Brazil, throughout the Caribbean and along the U.S. coasts from Texas to North Carolina.

Rufa red knots breed in the tundra of the central Canadian Arctic from northern Hudson Bay to the southern Queen Elizabeth Islands.

The largest concentration of rufa red knots is found in late May and early June in Delaware Bay, where the birds stop to gorge themselves on the eggs of spawning horseshoe crabs; a spectacle that also draws thousands of bird-watching tourists to the area.

In just a few days, the birds nearly double their weight to prepare for the final leg of their long journey to the Arctic.

Red knots winter and migrate in large flocks containing hundreds of birds. While one can guess at some of the benefits of traveling in large flocks, such as protection from predators, there is also a downside — susceptibility to habitat change and loss, oil spills, toxins, red tides, diseases, collisions with wind turbines, storms and hunting.

To endure their long journeys, red knots undergo extensive physical changes. Flight muscles enlarge, while leg muscles shrink. Stomachs and gizzards decrease, while fat mass increases by more than 50 percent.

Because of these physical changes, knots arriving from long migration flights are not able to feed maximally until their digestive systems regenerate, a process that may take several days. Thus, migrating birds require stopover habitats rich in easily digested foods — with thin or no shells — to gain enough weight to fuel the next flight.

In spring, migrating knots follow a northward “wave” in quality prey — timing their stopovers to take advantage of readily digestible food resources like horseshoe crab eggs. Red knots arrive at stopovers areas very thin, sometimes emaciated.

They must eat constantly to gain enough weight to continue their journeys, adding up to 10 percent of their body weight each day and nearly doubling their body weight during some stopovers.

Since the 1980s, the knot’s population has fallen by about 75 percent in some key areas, largely because of declines in one of its primary food resources – horseshoe crab eggs in Delaware Bay, an important migratory stopover site. Although this threat is now being addressed by extensive state and federal management actions, other threats, including sea level rise, some shoreline projects and coastal development, continue to shrink the shorebird’s wintering and migratory habitat.

Changing climate conditions are also altering the bird’s breeding habitat in the Arctic and affecting its food supply across its range, in particular through climate-driven mismatches in migration timing that affect the peak periods of food availability. The bird must arrive at Delaware Bay at exactly the time when horseshoe crabs are laying their eggs.

Unfortunately, this hearty shorebird is no match for the widespread effects of emerging challenges like climate change and coastal development, coupled with the historic impacts of horseshoe crab overharvesting, which have sharply reduced its population in recent decades.

In December 2014, the U.S. Fish and Wildlife Service announced federal protection for the rufa red knot, designating it as threatened under the Endangered Species Act. A “threatened” designation means a species is at risk of becoming endangered throughout all or a significant portion of its range.

International, state and local governments; the conservation community; beachgoers; and land managers are working together to ensure red knots have safe areas to winter, rest and eat during their long migrations. These partners are helping in a variety of ways, including collecting data to better understand these birds, managing the harvest of horseshoe crabs — which are caught for use as bait in conch and eel pots — reducing disturbance to key coastal habitats, and improving the management of hunting outside the United States.

For information about the life history, migration and conservation of rufa red knots go to www.fws.gov/northeast/redknot/.