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The largest calidridine sandpiper of North America and largest species in the genus Calidris, exceeded in size only by the Great Knot (C. tenuirostris), the Red Knot is colorful in breeding plumage, changing to dull gray in Basic (winter) plumage with few distinct markings. This species is a Holarctic breeder, mainly in middle- and high-arctic zones, with 3 races (islandica, rufa, and roselaari) distributed in the Nearctic from Greenland to northern Alaska. The nominate subspecies nests in central Russia (e.g., the northern Taymyr Peninsula), C. c. rogersi on the New Siberian Islands and in eastern Russia (e.g., Chukchi Peninsula), and C. c. roselaari on Wrangel Island and in northern Alaska. This account focuses largely on the Western Hemisphere subspecies.
Red Knots are noted for their extraordinary long-distance migrations, which for rufa includes travel between arctic tundra and marine wintering habitats in Tierra del Fuego, 15,000 km away in South America. This knot is also known for gathering in huge flocks at migration staging areas, where individuals fatten before embarking on long-distance, nonstop flights, some exceeding 2,500 km. Less well known in North America, but providing curious contrasts, are the migrations of C. c. islandica across the Atlantic from breeding areas in eastern arctic Canada and northern Greenland to coastal wintering habitats in the North Sea region of Europe (roughly 4,200 km from breeding areas). Although poorly known, C. c. roselaari evidently migrates from Alaska’s Seward Peninsula and North Slope (and perhaps also easternmost Siberia) breeding areas to Caribbean and Gulf of Mexico coastal wintering areas.
Population sizes for knots are perhaps better known than for any other of North America’s shorebirds, except endangered species. Estimates for the 3 races combined are fewer than a million birds; Alaskan roselaari are in the tens of thousands, rufa are in the low hundreds of thousands, and islandica are in the middle hundreds of thousands.
Red Knots are monogamous and single-brooded, and like most other northern shorebirds they typically lay a 4-egg clutch. Their courtship is accompanied by elaborate flight, ground, and vocal displays. For nesting, the species prefers drier tundra such as sparsely vegetated hillsides.
Red Knots are principally marine shorebirds in the nonbreeding season, when they ordinarily feed on small marine mollusks, especially bivalves that they swallow whole. An exception to this, and something for which rufa is perhaps best known, is huge flocks that gather during spring migration on the mid-Atlantic Coast (Delaware Bay) to gorge on eggs of horseshoe crabs (Limulus) that come ashore to lay in late May. Between mid-July and mid-August (south migration), most U.S. Atlantic coastal staging sites of the Red Knot are tidally flooded peat banks where individuals can find one of their favorite prey, the spat (diminutive form of the adults) of mussels (Mytilus edulis).
Research on Red Knots has focused primarily on distribution and taxonomy, long-distance migration, breeding behavior, and foraging. The species currently consists of 5 recognized subspecies (Engelmoer and Roselaar 1998) that breed around arctic polar regions of the earth, each showing slightly different physical features but having strikingly different migration strategies (Piersma and Davidson 1992). The species is thus an apt one for studying the development of avian migration. It breeds in some of the coldest regions of the earth (Tomkovich and Soloviev 1996) and spends winter in some of the hottest (Wolff and Smit 1990). It has been a key species for understanding relationships between ecological physiology and migration energetics (Gudmundsson et al. 1991; Piersma et al. 1994, 1995a) and development of migration strategies (Piersma 1991, 1998; Wiersma and Piersma 1994). In the Western Hemisphere this knot has been a focal research species for development of conservation strategies for long-distance migrants (Myers et al. 1987, Harrington 1996, Piersma and Baker 2000), including development of the Western Hemisphere Shorebird Reserve Network (Bildstein et al. 1991).