Calidris canutus rufa is a subspecies of red knot occurring in the western hemisphere. Birds are characterized by a stocky body, a short black bill, dark olive to gray legs, and white rump coloration that does not extend up the back (Terres 1980; Paulson 2005). Plumage and body coloration varies with age and season, and descriptions are divided accordingly below.
The face and underparts are reddish, with brownish black or mottled black and white markings on the majority of the body. The belly is marked with variable amounts of white, and the posterior sides and undertail bear sparse black bars. Males are generally more colorful than females, with more red coloration on the upperparts, smaller white patches on the belly and fewer bar markings on the breast (Peterson 1980; Terres 1980; Paulson 2005).
Plumage is white or light brown below, with gray-brown darkly streaked upperparts. Legs are greenish and paler than those of breeding adults (Peterson 1980; Terres 1980; Paulson 2005).
Legs are greenish and often paler than those of the adult forms. Plumage coloration is similar to non-breeding adults, but with the breast often more prominently marked with fine brown streaks instead of dots. Younger juveniles may have buff underparts (Peterson 1980; Terres 1980; Paulson 2005).
Potentially Misidentified Species
The red knot is similar in appearance to a few other species inhabiting the IRL and surrounding areas, including: two species of dowitcher belong to the genus Limnodromus; the sanderling, Calidris alba; and the black-bellied plover, Pluvialis squatarola (Paulson 2005). Both species of dowitcher occurring locally have longer bills than the red knot, generally grayer back coloration, duller red underparts, and smaller white markings on the underparts. Sanderlings are much smaller and black-bellied plovers are larger than C. canutus rufa.
Flight Patterns & Locomotion
While in flight, red knots form close flocks with a uniform twisting motion that reveals their gray backs followed by the red of their breasts. Flight speeds of some individuals have been recorded up to 38 mph (Terres 1980).
HABITAT AND DISTRIBUTION
Red knots in the western hemisphere complete one of the longest annual migrations in the avian world, breeding in the central Canadian Arctic and traveling southward to winter mainly in Brazil (Diaz et al. 2005; Niles et al.2008). During migration, birds may stopover in many locations, including California, the Gulf coasts of Texas and Florida, and along the Atlantic coast of the U.S. from Florida to Maine.
Wintering populations of red knots have been documented mostly from the northern IRL, including the Kennedy Space Center and the Merritt Island National Wildlife Refuge (NWR) (Niles et al. 2008). Birds are found on tidal flats in the lagoon and in the swash zone of sandy beaches exposed to the coastal waters of the adjacent Atlantic Ocean (e.g. Paulson 2005; Niles 2008).
LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan
The red knot is the largest of the North American beach sandpipers, measuring approximately 24 cm in length with a wingspan up to 51 cm (Terres 1980; Paulson 2005). Sexes appear similar, but females are often slightly larger. Lifespan varies with environmental conditions and other factors, but individuals have been documented to live to at least 13 years of age under certain conditions (Terres 1980).
Surveys of wintering red knot populations in Florida during the 2005-2006 season revealed a statewide abundance of approximately 4,000 individuals (Niles et al. 2008). Detailed records for sites within the IRL are scarce, but results from a 1993-1994 census found a maximum of 164 birds within the Merritt Island NWR in the northern lagoon, one of the few sites surveyed in the region (Niles et al. 2008).
Nests of the red knot are typically built on dry, rocky arctic tundra at high elevations (Terres 1980). Birds construct nests around clumps of lichens and scant vegetation, and among rocky outcroppings on hills and ridges. Eggs are laid in June and July, about four per nest, light olive in color and spattered with brown markings. Both parents incubate the eggs until they hatch, a period of about 21-22 days, and most young are capable of flight approximately 18 days after hatching.
Red knots emit a variety of calls, depending on the circumstance. During flight, vocal communication consists of a single croak, a honking knut or tlu tlu, or a mellow wah-quoit or cur-ret. Nesting males in the arctic breeding grounds are reported to call poor-me, while birds in flight over nesting areas emit a repeated whistle of pooo-lee pooo-lee pooo-lee followed by a couple of short trills (Peterson 1980; Terres 1980; Paulson 2005).
The natural range of the red knot extends from arctic to tropical climate zones, suggesting that the species has a broad thermal tolerance, which is regulated in part by seasonal migration (see 'Habitat & Distribution' above).
Little information is available concerning habitat salinity preferences of red knots. However, the species is most commonly found in brackish to marine coastal ecosystems where it feeds primarily on aquatic organisms that are acclimated to a particular salinity range. In the arctic, some breeding birds in search of prey will frequent freshwater sources near their nests (Terres 1980).
During their annual northward migration, red knots stop on the Delaware coast where they feed primarily on the eggs of the horseshoe crab, Limulus polyphemus (Niles et al. 2008). This food resource provides fats and protein essential for the birds’ long journey to their arctic breeding grounds. In recent years, Delaware horseshoe crab populations have declined from increased bait fishing pressure, leading to negative impacts on the red knot. See 'Special Status' below. On beaches, the red knot uses its bill to probe the sand for molluscan prey, including periwinkles and coquina clams of the genus Donax (Terres 1980). Birds inhabiting tidal flat and salt marsh ecosystems feed on a variety of items, including: small fishes, marine worms, seeds from seagrasses and marsh plants, and a host of insects (Terres 1980).
Little information is available concerning predators of the red knot. However, it is likely that the small size of the species allows it to be preyed upon by a variety of large organisms, including mammals, alligators and birds of prey.
Like many other bird species, the red knot acts as a terminal or final host for several parasites acquired from a variety of prey items, including the parasitic worms, Bartolius pierrei (Cremonte 2004), Skrjabinocerca canutus, Viktorocara capillaries, and V. limosae (Diaz et al. 2005).
Although there are no obligate associations documented between the red knot and other species, C. canutus rufa is commonly found alongside other organisms from the tidal flats and sandy beach habitats in which it resides. For more extensive information on these ecosystems and associated species found in and around the IRL, please visit the Tidal Flat and Beach Habitat pages.
The U.S. Fish and Wildlife Service listed the red knot as a candidate for protection under the Endangered Species Act in 2005 (USFWS 2010).
Threats & Conservation
As the main food resource for northward migrating red knots in the western hemisphere, threats to the abundance and quality of eggs from Delaware horseshoe crab populations have lead to negative impacts on bird populations (Niles et al. 2008). These effects possibly include delayed recovery time during the migratory stopover and diminished reproductive capacity once the birds reach their breeding grounds. Delaware has taken several steps in recent years to combat these potential threats to migrating red knots, including beach closures to quell human disturbances, exclosures to reduce competition from gulls, harvest restrictions to improve horseshoe crab populations, and the designation of a sanctuary at the mouth of Delaware Bay.
In Florida, wintering red knot populations are potentially threatened by human disturbances that alter beach topography, including shoreline hardening, dredging, deposition and raking from beach nourishment programs (Niles et al. 2008). Although these activities require permitting, the exact locations and degrees of beach alteration are rarely documented. Also lacking are data concerning the effects of these actions on local shorebird populations, although the impacts are thought to be significant.
Some management efforts have been made to protect Florida red knot populations from human disturbance by limiting or excluding access to roosting sites, including those at Shell Key and Caladesi Island State Park in Pinellas County, Passage Key National Wildlife Refuge (NWR) in Manatee County, Merritt Island NWR and Kennedy Space Center in Brevard County, and Ding Darling NWR in Lee County (Niles et al. 2008). Calls have been made for more active restoration efforts for red knots in the state, including banding wintering birds, collecting biometric and genetic data, and surveying previously banded individuals to determine migration patterns and survivorship.
Cremonte, F. 2004. Life cycle and geographic distribution of the gymnophallid Bartolius pierrei (Digenea) on the Patagonian coast, Argentina. J. Nat. Hist. 38: 1591-1604.
Diaz, JI, Cremonte, F, Navone, GT & S Laurenti. 2005. Adults and larvae of Skrjabinocerca canutus n. sp. (Nematoda: Acuariidae) from Calidris canutus rufa (Aves: Scolopacidae) on the southern Southwest Atlantic coast of South America. Syst. Parasitol. 60: 113-123.
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Peterson, RT. 1980. A Field Guide to the Birds: A Completely New Guide to All the Birds of Eastern and Central North America. Houghton Mifflin. Boston, MA. USA. 384 pp.
Terres, JK. 1980. The Audubon Society Encyclopedia of North American Birds. Alfred A. Knopf. New York. USA. 1109 pp.
USFWS. 2010. The red knot (Calidris canutus rufa). U.S. Fish & Wildlife Service. Online at http://www.fws.gov/northeast/redknot/ (Date accessed 08/12/2010).