Rufa Red Knot

A singing male’s flight songs are a flutelike poorr-mee that becomes a series of poorr-poor.  

Birds flushed from the nest utter a quick whit-whit. Males in conflict give a chatter yodel, written yeh-yeh-yeh, while males courting females produce a high weeee. During most of the year, knots that are foraging or flushed simply give a nasal knutt.

The rufa red knot is stocky, medium-sized shorebird with relatively short bill and legs. They measure about 9 to 11 inches (in) (23 to 28 centimeters (cm)) in length, with a wingspan up to 20 inches (50.8 cm). Red knots have a proportionately small head, small eyes, and short neck, and a black bill that tapers from a stout base to a relatively fine tip. The bill length is not much longer than head length.

Body mass varies seasonally, with lowest average mass during early winter (4.4 ounces (oz); 125 grams (g)) and highest weights during spring (7.2 oz; 205 g) and fall (6.1 oz; 172 g), usually just before departure on a long migration flight. 

The red knot is easily recognized during the breeding season by its distinctive rufous (red) plumage. The face, prominent stripe above the eye, breast, and upper belly are a rich rufous-red to a brick or salmon red, sometimes with a few scattered light feathers mixed in. The feathers of the lower belly and under the tail are whitish with dark flecks. Upperparts are dark brown with white and rufous feather edges; outer primary feathers are dark brown to black. Females are similar in color to males, though the rufous colors are typically less intense, with more buff or light gray on the dorsal (back) parts. Breeding plumage can be observed in certain migration stopover habitats as well as on the arctic breeding grounds. 

Nonbreeding plumage is dusky or pale ashy gray above, from crown to rump, with feathers on back narrowly edged with white. The underparts are whitish, the breast lightly streaked and speckled, and the flanks narrowly barred with gray. Juveniles resemble nonbreeding adults, but the feathers of the scapulars (shoulders) and wing coverts (small feathers covering base of larger feathers) are edged with white and have narrow, dark bands, giving the upperparts a scalloped appearance.

The legs are typically dark gray to black, but sometimes greenish in juveniles or older birds in nonbreeding plumage.

Across all six subspecies, red knots are specialized molluscivores, eating hard-shelled mollusks, sometimes supplemented with easily accessed softer invertebrate prey, such as shrimp- and crab-like organisms, marine worms, and horseshoe crab (Limulus polyphemus) eggs. The mollusk prey is swallowed whole and crushed in the gizzard, which in the red knot is the largest (relative to body size) among any shorebird species evaluated. Large gizzards are among this species’ adaptations to a mollusk diet, allowing red knots to grind the hard shells of its prey. Red knots prefer thin-shelled over thick-shelled prey species because they are easier to digest and provide a more favorable meat-to-mass ratio (higher prey quality). From studies of other subspecies, some researchers concluded that red knots cannot ingest prey with a circumference greater than 1.2 inches. For rufa red knots, prey lengths of 4 to 20 millimeters (mm) have been observed. Foraging activity is largely dictated by tidal conditions, as red knots rarely wade in water more than 2 to 3 centimeters (cm) deep. Due to bill morphology, the red knot is limited to foraging on only shallow-buried prey, within the top 2 to 3 cm of sediment. Along the U.S. Atlantic and Gulf coasts, Donax and Mulinia clams and blue mussel (Mytilus edulis) spat are key prey items for the rufa red knot. A prominent departure from typical prey items occurs each spring when rufa red knots feed on the eggs of horseshoe crabs, particularly (but not exclusively) in the key Delaware Bay migration staging area. In Delaware Bay, horseshoe crab eggs are a superabundant source of easily digestible food, and horseshoe crab eggs are a preferred food item across many portions of the U.S. coast. Away from the coasts, the rufa red knot’s diet is poorly known but may include brine shrimp or insects.

Rufa red knots undertake long flights that may span thousands of miles without stopping. As red knots prepare to depart on long migratory flights, they undergo several physiological changes. Before takeoff, the birds accumulate and store large amounts of fat to fuel migration and undergo substantial changes in metabolic rates. In addition, the leg muscles, gizzard, stomach, intestines, and liver all decrease in size, while the pectoral muscles and heart increase in size. Due to these physiological changes, red knots arriving from lengthy migrations are not able to feed maximally until their digestive systems regenerate, a process that may take several days. 

Delaware Bay serves as the principal spring migration staging area for the rufa red knot because of the abundance and availability of horseshoe crab eggs. Delaware Bay provides the final Atlantic coast stopover for a significant majority (50 to 80 percent) of the red knot population making its way to the arctic breeding grounds each spring. Rufa red knots stopping in Delaware Bay depend on horseshoe crab eggs to achieve remarkable rates of weight gain. Weight gains recorded on the Delaware Bay are the most rapid of any recorded for all known stopover populations of red knots in the world and are among the highest rates observed in the animal kingdom. Although a single horseshoe crab egg contains a very small amount of energy, eggs are present in such superabundance that birds can eat enough in two weeks to nearly double their weights.

On the breeding grounds, the rufa red knot's diet consists mostly of terrestrial invertebrates such as insects and other arthropods. However, early in the breeding season, before insects and other macroinvertebrates are active and accessible, red knots will eat grass shoots, seeds, and other vegetable matter.

Each year, some rufa red knots make one of the longest distance migrations known in the animal kingdom, traveling up to 19,000 miles annually. Rufa red knots tend to migrate in single-species flocks with departures typically occurring in the few hours before twilight on sunny days. Size of the departing flocks tends to be large (greater than 50 birds). Observations of a different red knot subspecies found mean flock sizes were 100 to 200 individuals, departure of flight formations was typically during the afternoon or evening and during rising or high tide, and departures had significant differences in daily timing between seasons associated with the tidal cycle. Rufa red knots are thought to migrate during both night and day.

Unknown numbers of nonbreeding rufa red knots remain south of breeding grounds during the breeding season, and many, but not all, of these knots are one-year-old (i.e., immature) birds. Little information is available about these non-breeding birds. Nonbreeding rufa red knots, usually individuals or small groups, have been reported during June along the U.S. Atlantic and Gulf coasts, with smaller numbers around the Great Lakes and Northern Plains in both the United States and Canada. All juveniles of the Tierra del Fuego wintering region are thought to remain in the Southern Hemisphere during their first year of life, possibly moving to northern South America.

In nonbreeding habitats, rufa red knots require sparse vegetation and open landscapes to avoid predation. The vulnerability of red knots, and their reactivity to perceived predation danger, may be related to their field of vision. Studies from other subspecies found that red nots do not show comprehensive panoramic vision as found in some other tactile-feeding shorebirds, but have a binocular field surrounding the bill and a substantial blind area behind the head. This visual system may be a tradeoff for switching to more visually guided foraging (i.e., insects) on the breeding grounds. However, this forward-focused visual field leaves red knots vulnerable to aerial predation, especially when using tactile foraging in nonbreeding locations where predation by falcons is an important selection factor.

Rufa red knots show moderate fidelity to particular migration flyways and staging areas between years. Evidence from several coastal areas suggest that rufa red knots often rely on clusters of suitable habitats during migration -- such habitat complexes may also be important for the resiliency of red knots. Fidelity to a wintering region is extremely high, with only a few isolated examples of birds found in one region subsequently observed in a different region during the core winter months. However, within a wintering region, birds are known to shift around on local and regional scales. On the breeding grounds, little is known about mate fidelity between years. There is only limited evidence of nest-site fidelity in rufa red knots despite studies of other subspecies suggest breeding site fidelity is high, especially in males. However, only a small number of rufa red knot nests, from a limited area of the breeding range, have been studied to date.

Territorial displays begin and pair bonds form soon after rufa red knots arrive on the breeding grounds. One study on Southampton Island (Canada) found that nests were spaced about 1 kilometer (km) apart; birds stayed within 2 km of the nest and appeared to defend an area of 0.5 km around the nest. Both sexes participate equally in egg incubation. Young are precocial, leaving the nest within 24 hours of hatching and foraging for themselves. After the eggs hatch, rufa red knot chicks and adults quickly move away from high nesting terrain to lower, freshwater wetland habitats. Females are thought to leave the breeding grounds and start moving south soon after the chicks hatch in mid-July. Thereafter, parental care is provided solely by the males, but about 25 days later males also abandon the newly fledged juveniles and move south. Not long after, they are followed by the juveniles.

Rufa red knots begin their life cycle as eggs laid in the Arctic tundra. Chicks hatch in about three weeks and fledge (learn to fly) around 18-20 days old. With the adult birds having already departed the Arctic, the fledged chicks must undertake their first migration on their own. These "hatch-year" birds migrate south to the same wintering regions as adults (see Geography), but they do not typically migrate all the way to Tierra del Fuego. After their first winter, juvenile birds migrate north but stop short of the breeding grounds, spending their first summers in coastal areas from South America to southern Canada before migrating south again to their wintering grounds. During their second spring, most rufa red knots undertake their first northbound migration all the way to the Arctic breeding grounds, a trip that most birds then repeat each year. However, some adult birds skip breeding in some years, remaining in coastal portions of the nonbreeding range. 

Red knots perform flight and vocal displays during courtship. Knots are monogamous and produce one brood each year. Males arrive at the northern breeding grounds before the females to prepare nesting sites. In June or July, the female lays a clutch, typically with four eggs, that both parents tend. Chicks leave the nest soon after hatching, accompanied by their parents to foraging areas. Females are thought to leave the breeding grounds and start moving south soon after the chicks hatch in mid-July. Thereafter, parental care is provided solely by the males, but about 25 days later (around August 10) males also abandon the newly fledged chicks and move south. Not long after, they are followed by the juveniles.

The oldest Calidris canutus of any subspecies recorded worldwide was estimated to be 25 years old when recaptured. The oldest recorded rufa red knot (marked with the famous orange flag code B95) was at least 21 years old when last resighted in spring 2014. Although these records indicate that the potential lifespan is considerable, the average life span is thought to be much less. Based on estimated survival rates for a stable population, one assessment found that few rufa red knots live for more than about 7 years. No rigorous longevity analysis has been done since the 7-year estimate, but an update may be warranted based on a number of birds known (from resightings) to be considerably older than 7 years of age.

Coastal habitats used by rufa red knots in migration and wintering areas are similar in character: generally coastal marine and estuarine habitats with large areas of exposed intertidal sediments. Migration and wintering habitats include both high-energy ocean- or bay-front areas, as well as tidal flats in more sheltered bays and lagoons. Preferred wintering and migration habitats are muddy or sandy coastal areas, specifically, bays and estuaries, tidal flats, and unimproved tidal inlets. Along the U.S. Atlantic coast, dynamic and ephemeral features are important rufa red knot habitats, including sand spits, islets, shoals, and sandbars, features often associated with inlets. In many wintering and stopover areas, quality high-tide roosting habitat (i.e., close to feeding areas, protected from predators, with sufficient space during the highest tides, free from excessive human disturbance) is limited. In nonbreeding habitats, rufa red knots require sparse vegetation and open landscapes to avoid predation.
 

Rufa red knots and other shorebirds that are long-distance migrants must take advantage of seasonally abundant food resources at migration stopovers to build up fat reserves for the next nonstop, long-distance flight. Although foraging rufa red knots can be found widely distributed in small numbers within suitable habitats during the migration period, the birds tend to concentrate in those areas where abundant food resources are consistently available from year to year. The spatial distribution of red knots in Argentina, Georgia, South Carolina, Virginia, the Atlantic coast of New Jersey, and Delaware Bay stopover areas has been correlated with the distribution of the primary prey species in these areas. 

Available information suggests that rufa red knots use inland saline lakes as stopover habitat in the Northern Great Plains. We have little information to indicate whether or not red knots may also use inland freshwater habitats during migration, but data suggest that certain freshwater areas (e.g., wetlands, riverine sandbars) may warrant further study as potential stopover habitats. Available data indicate that small numbers of rufa red knots sometimes use manmade freshwater habitats (e.g., impoundments) along inland migration routes.

Rufa red knots generally nest in dry, slightly elevated tundra locations, often on windswept slopes with little vegetation. The best available information indicates nest sites are within 600 feet of a freshwater wetland, and, although nests are typically within 31 miles of the ocean, knots do not typically use saltwater habitats on the breeding grounds. However, the total number of rufa red knot nests studied to date is small and highly concentrated on just one island; thus, it is possible that a they use a greater diversity of nesting and foraging habitats across the breeding range than have yet been documented. Nests may be scraped into patches of mountain avens (Dryas octopetala) plants, or in low spreading vegetation on hummocky ground containing lichens, leaves, and moss. After the eggs hatch, rufa red knot chicks and adults quickly move away from high nesting terrain to lower, freshwater wetland habitats.