The red knot, also known as Calidris canutus, is a medium-sized shorebird that lives in the tundra and Arctic Cordillera regions in the far north of Canada, Europe, and Russia. It is one of the largest birds in the Calidris sandpiper family, second only to the great knot. Scientists have identified six different subspecies of this bird.

The red knot’s diet changes with the seasons. During the breeding season, it eats mostly arthropods and larvae. At other times of the year, it feeds on hard-shelled mollusks in different areas. North American red knots migrate to coastal regions in Europe and South America, while Eurasian red knots spend the winter in Africa, Papua New Guinea, Australia, and New Zealand. These birds often form large groups when they are not breeding.

Taxonomy, systematics, and evolution

The red knot was first named by Carl Linnaeus in his 1758 edition of Systema Naturae as Tringa canutus. One idea is that the bird’s name and species label may come from King Cnut, as the bird feeds near the water’s edge, similar to a story about King Cnut and the tide. However, there is no clear historical evidence to support this. Another possibility is that the name is onomatopoeic, meaning it sounds like the bird’s grunting call.

The red knot and the great knot were originally the only two species in the genus Calidris. Later, many other sandpiper species were added to this group. A 2004 study showed that the genus Calidris is polyphyletic, meaning it includes species that are not closely related. The closest relative to the two knot species is the surfbird (Calidris virgata).

There are six subspecies of the red knot, listed in order of size:
• C. c. roselaari (Tomkovich, 1990) – (largest)
• C. c. rufa (Wilson, 1813)
• C. c. canutus (Linnaeus, 1758)
• C. c. islandica (Linnaeus, 1767)
• C. c. rogersi (Mathews, 1913)
• C. c. piersmai (Tomkovich, 2001) – (smallest)

Studies using genetic differences and models of ancient climate changes suggest that C. c. canutus is the oldest population, splitting from others about 20,000 years ago (with a 95% confidence range of 60,000–4,000 years ago). Two distinct groups of red knots, one breeding in America and the other in Siberia, began to separate about 12,000 years ago (with a 95% confidence range of 45,000–3,500 years ago).

Distribution and migration

During the breeding season, the red knot is found in the high Arctic. It then travels to coasts around the world to spend the winter between 60° N (Shetland) and 55° S (Tierra del Fuego). The red knot is known for one of the longest migrations of any landbird. Some birds of the subspecies C. c. rufa travel more than 14,000 km (9,000 mi) each year from the Arctic to the southern tip of South America and return in the opposite direction. Scientists are still learning the exact paths these birds take and where they spend the winter. The nominate subspecies, C. c. canutus, breeds in the Taymyr Peninsula and possibly Yakutia, and migrates to Western Europe before wintering in western and southern Africa. C. c. rogersi breeds in the Chukchi Peninsula in eastern Siberia and winters in eastern Australia and New Zealand. A few C. c. rogersi (and possibly another subspecies, C. c. piersmai) spend the winter on mudflats in the Gulf of Mannar and along the eastern coast of India. The recently identified subspecies C. c. piersmai breeds in the New Siberian Islands and winters in north-western Australia. C. c. roselaari breeds in Wrangel Island in Siberia and north-western Alaska and appears to winter in Florida, Panama, and Venezuela. C. c. rufa breeds in the Canadian low Arctic and winters in South America. C. c. islandica breeds in the Canadian high Arctic and Greenland and migrates through Iceland to winter in Western Europe.

Birds that winter in west Africa limit their daily foraging to a small area of 2–16 km (0.77–6.18 sq mi) where the sea meets the land and rest at one location for several months. In temperate regions like the Wadden Sea, red knots change roosting sites weekly, and their feeding range can cover up to 800 km (310 sq mi) in a single week.

The red knot lives longer than most birds of its size. A bird tagged in Brora, northern Scotland, in October 1978 (when it was 3–4 months old) was seen alive again in Den Helder, the Netherlands, in February 2006—27 years, 3 months, and 29 days later. Another bird, tagged "B95" and called "Moonbird," from the subspecies C. c. rufa, was at least 20 years old when it was last seen in May 2014.

Description and anatomy

The red knot is the third or fourth largest bird in the Calidris sandpiper group, following the great knot, male ruff, and the similarly sized surfbird. It measures 23–26 cm (9.1–10.2 in) in length with a wingspan of 47–53 cm (19–21 in). Its body shape is typical for its genus, with a small head and eyes, a short neck, and a slightly tapered bill that is no longer than its head. It has short, dark legs and a medium, thin, dark bill. During winter, its feathers become uniformly pale grey and look similar in both males and females. In breeding season, its feathers are mottled grey on top, with a cinnamon face, throat, and breast, and a light-colored rear belly. Female red knots have similar breeding plumage to males, but their feathers are slightly lighter, and the line around their eyes is less clear. The subspecies C. c. canutus, C. c. islandica, and C. c. piersmai have darker feathers than other subspecies. C. c. rogersi has a lighter belly than C. c. roselaari or C. c. piersmai, and C. c. rufa has the lightest overall feathers. The change from breeding to winter plumage begins at the breeding site but is most noticeable during migration. The change to breeding plumage starts before migration to the breeding grounds but occurs mostly during the journey.

The red knot’s large size, white wing bar, grey rump, and tail make it easy to spot in flight. When feeding, its short, dark green legs give it a low-slung appearance. When foraging alone, it rarely calls, but when flying in flocks, it makes a low, single-syllable "knutt" sound. During migration, it produces a two-syllable "knuup-knuup" call. It breeds in moist tundra from June to August. The male’s display song is a fluty "poor-me," and its display involves flying high, flapping its wings rapidly, and then tumbling to the ground with wings raised upward. Both males and females incubate eggs, but the female leaves the care of the young to the male after the eggs hatch.

Juvenile red knots have distinctive submarginal lines and brown feathers during their first year. During the breeding season, males can be hard to distinguish from females (less than 80% accuracy compared to genetic testing) based on the more even red color on their underparts that extends toward the rear of their belly.

The weight of red knots varies by subspecies, ranging from 100 to 200 grams (3.5 to 7.1 ounces). Before migration, red knots can double their weight. Like many migratory birds, they shrink their digestive organs before migration. This change is less extreme than in species like the bar-tailed godwit, likely because red knots have more feeding opportunities during migration. Red knots can also adjust the size of their digestive organs seasonally. Their gizzard becomes thicker when eating harder foods in winter and thinner when eating softer foods in breeding areas. These changes can happen very quickly, sometimes within six days.

Behaviour

Red knots eat different foods depending on where they are. On breeding grounds, they mostly eat spiders, insects, larvae, and some plants by pecking at the surface. During winter and migration, they eat hard-shelled animals like mussels, snails, and small crabs. These prey are swallowed whole and crushed in their stomachs.

When feeding in mudflats during winter and migration, red knots use their sense of touch to find prey hidden in the mud. They probe shallowly into the mud while walking along the shore. When the tide is going out, they often peck at the surface or dig into soft mud with their bills, inserting them about 1 cm deep. In Europe, their favorite prey is a type of bivalve called Macoma, which they swallow whole and crush in their gizzard. In Delaware Bay, they eat many horseshoe crab eggs during spring, which are easy to digest and available when the birds arrive. They can sense buried mollusks by feeling changes in water pressure through special sensors in their bills. During the breeding season, they switch to using sight to find prey that are not hidden, pecking at them instead. Pecking is also used to eat surface foods like horseshoe crab eggs in winter and migration areas.

Red knots are territorial and mate with one partner during the breeding season. It is not known if pairs stay together between seasons. Males in Russia return to the same breeding areas each year, but it is unclear if they stay in the same territory. Males arrive before females and start defending their territory. As soon as they arrive, they begin showing off and fighting other males to protect their space.

Red knots build their nests on the ground near water, usually inland. The nest is a shallow hole lined with leaves, lichens, and moss. Males create three to five nest sites before females arrive. The female lays three to four eggs over six days. Each egg is about 43 mm long and 30 mm wide, with a light to dark olive color and a slight shine. Both parents take turns incubating the eggs, spending equal time on them. When not sitting on the eggs, one parent joins flocks of other red knots to find food. Incubation lasts about 22 days. Early in this time, adults may leave the nest quickly if people are nearby and not return for hours. Later, they stay on the eggs. The eggs hatch at the same time. The chicks are born with downy feathers that help them blend in. Within a day of hatching, the chicks and parents move away from the nest and start foraging together. The female leaves before the young can fly, while the male stays. After the young fledge, the male migrates south, and the young travel on their own for the first time.

Relationship with horseshoe crab

The red knot is one of the longest-distance migrants in the animal kingdom. It depends on the same stopping sites each year during its migration to and from breeding areas. These birds travel in larger groups than most shorebirds, flying about 9,300 miles from south to north each spring and repeating the journey in the opposite direction each autumn. During the northern hemisphere’s winter, red knots live in Tierra del Fuego, South America. In the summer, they migrate to breeding areas on islands and mainland regions north of the Arctic Circle. Their long journeys are divided into segments of about 1,500 miles, ending at staging areas they visit yearly. The Delaware Bay is the most important stopover for red knots because they eat large amounts of horseshoe crab eggs there, which provide the energy they need for migration. The timing of red knots’ arrival matches the annual spawning of horseshoe crabs in the Delaware Bay. Studies show that the number of horseshoe crab eggs on beaches like those in Delaware Bay influences where red knots travel and how they are distributed. Other stopover sites in the United States include islands near Massachusetts, Virginia, South Carolina, and Georgia. Fewer red knots travel overland and spend winters along the Gulf Coast. Stopover areas on this route include the Mississippi River drainage, saline lakes in the northern U.S., and plains in southern Canada.

Before migration, red knots undergo physical changes to prepare for their long journey. Their flight muscles grow larger, while leg muscles shrink. Their stomach and gizzard sizes decrease, but fat stores increase by more than 50 percent. When they arrive at stopover sites, they are very thin. Because their gizzards are smaller, they eat fewer hard foods and prefer soft, nutritious horseshoe crab eggs. Since the timing of migration matches the release of crab eggs, these eggs are easily found and digested, saving the birds energy. The availability of horseshoe crab eggs in these areas supports the survival of red knots. During stopovers lasting about 10–14 days, red knots eat constantly to build up body fat, which can double their body weight and help them complete the rest of their journey. The large number of horseshoe crabs in Delaware Bay makes it the most important stopover site, supporting an estimated 50 to 80 percent of all migrating rufa red knots each year.

The survival of red knots is closely connected to the number of horseshoe crab eggs available. In the early 20th century, horseshoe crabs were harvested for fertilizer and animal feed. Today, they are still collected for use as bait by U.S. fishing companies. Overharvesting reduced crab populations, and red knot numbers in Tierra del Fuego (winter) and Delaware Bay (spring) dropped by about 75 percent between the 1980s and 2000s. In the 1990s, the Atlantic States Marine Fisheries Commission introduced measures to protect crab populations, including state-by-state harvest limits and the creation of a 1,500-square-mile horseshoe crab sanctuary near the mouth of Delaware Bay. These efforts, along with new bait conservation methods, have reduced the use of horseshoe crabs for bait. As a result, red knot populations have stabilized.

Status

The red knot lives in a large area, covering about 100,000 to 1,000,000 square kilometers (39,000 to 386,000 square miles), and has a population of around 1.1 million birds. This species does not seem to meet the IUCN Red List criteria for population decline, which would require a drop of more than 30% in ten years or three generations. Because of this, it is classified as "least concern." However, some local populations have decreased, such as in the Dutch Wadden Sea, where digging up cockles (a type of shellfish) reduced the number of red knots that winter there. The quality of food at places where red knots stop during their migration is very important for their journey.

This species is protected by the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA). This agreement requires countries that signed it to control the hunting of these birds and their eggs, create protected areas to preserve their habitats, manage hunting practices, and track bird populations.

Red knot numbers have remained stable in recent years, but they are still relatively low.

Threats

Red knot populations are greatly affected by climate change because their breeding habitats in the middle and high Arctic are essential for survival. These birds breed and spend the winter in areas near the North Pole and along coasts, which are most impacted by climate change. Their nests are usually built on open ground in the tundra near water. Male red knots prepare 3–5 nest sites on dry and rocky areas of the tundra in upland regions, often near ridges and close to wetlands. Rising sea levels, coastal erosion, and warmer temperatures caused by global warming are destroying the best places for red knots to breed in Arctic coastal areas. Migratory animals have naturally adapted to different environments, but the rapid and complex changes from climate change are harmful to red knots' ability to survive. As Arctic breeding grounds warm, red knot body size has decreased, and fewer birds born in warmer years survive. In their wintering areas in the tropics, conditions have become more stable, leading to shorter bills (likely because stable environments support more successful growth and less variation within species). This makes it harder for red knots to reach their main food sources, such as mollusks buried deep in the ground, which reduces their access to food and increases the energy needed to find it.

Other threats, such as overharvesting of horseshoe crabs and climate change, are harming red knot populations. While climate change may be harder to address directly, reducing horseshoe crab harvesting and limiting human activities that disturb their food sources and breeding areas are effective ways to help protect red knots.

Conservation efforts

In 2003, scientists predicted that if the American rufa subspecies continued to decline at its current rate, it might become extinct by 2010. However, as of April 2011, the subspecies still exists. In New Jersey, state and local agencies are working to protect these birds by limiting the harvesting of horseshoe crabs and controlling access to beaches. In Delaware, a two-year ban on harvesting horseshoe crabs was introduced but canceled by a judge, who stated there was not enough evidence to support the ban’s potential impact on the fishing industry. In recent years, Delaware has allowed the harvesting of only male horseshoe crabs.

In late 2014, the red knot rufa was officially listed as a federally threatened species under the United States Endangered Species Act, which is the second-highest level of protection available for a subspecies. This decision followed many years of strong efforts by environmental groups and a lawsuit against the Department of the Interior for not properly protecting endangered species by failing to evaluate and list them. The reasons for the red knot rufa’s threatened status included habitat loss, the disappearance of important food sources, and threats from climate change and rising sea levels. These factors were all considered when the subspecies was listed.