Their diet varies according to season; arthropods and larvae are the preferred food items at the breeding grounds, while various hard-shelled molluscs are consumed at other feeding sites at other times.

North American breeders migrate to coastal areas in Europe and South America, while the Eurasian populations winter in Africa, Papua New Guinea, Australia, and New Zealand.

[8] A 2004 study found that the genus was polyphyletic and that the closest relative of the two knot species is the surfbird (currently Aphriza virgata).

Every year it travels more than 9,000 mi (14,000 km) from the Arctic to the southern tip of South America and repeats the trip in reverse.

[8] Small and declining numbers[13] of rogersi (but possibly of the later described piersmai) winter in the mudflats in the Gulf of Mannar and on the eastern coast[14] of India.

[16] C. c. roselaari breeds in Wrangel Island in Siberia and north-western Alaska, and it apparently winters in Florida, Panama and Venezuela.

Birds wintering in west Africa were found to restrict their daily foraging to a range of just 2–16 km2 (0.77–6.18 sq mi) of intertidal area and roosted a single site for several months.

The alternate, or breeding, plumage is mottled grey on top with a cinnamon face, throat and breast and light-coloured rear belly.

[19] The large size, white wing bar and grey rump and tail make it easy to identify in flight.

When foraging singly, they rarely call, but when flying in a flock they make a low monosyllabic knutt and when migrating they utter a disyllabic knuup-knuup.

In the breeding season the males can be separated with difficulty (<80% accuracy in comparison to molecular methods[20]) based on the more even shade of the red underparts that extend towards the rear of the belly.

[22][23] On the breeding grounds, knots eat mostly spiders, arthropods, larvae, and some plant material obtained by surface pecking, and on the wintering and migratory grounds they eat a variety of hard-shelled prey such as bivalves (including mussels),[24] gastropods and small crabs that are ingested whole and crushed by a muscular stomach.

[19] While feeding in mudflats during the winter and migration red knots are tactile feeders, probing for unseen prey in the mud.

When the tide is ebbing, they tend to peck at the surface and in soft mud they may probe and plough forward with the bill inserted to about 1 cm (0.39 in) in depth.

[25][26] In Delaware Bay, they feed in large numbers on the eggs of horseshoe crabs, a rich, easily digestible food source, which spawn just as the birds arrive in spring.

[27][28] They are able to detect molluscs buried under wet sand from changes in the pressure of water that they sense using Herbst corpuscles in their bill.

[31] Red knots travel “in larger flocks than do most shorebirds" flying “9300 miles from south to north every [northern hemisphere] spring and repeat the trip in reverse every autumn”.

Specifically, the Delaware Bay is the most vital migratory rest stop for the red knot, as much of their physiological demands are met by consuming the abundance of horseshoe crab eggs as their main food source during migration.

Stopover areas on this route are found in the Mississippi river drainage, Northern U.S. saline lakes, and plains in Southern Canada.

[1] However many local declines have been noted such as the dredging of intertidal flats for edible cockles (Cerastoderma edule) which led to reductions in the wintering of islandica in the Dutch Wadden Sea.

[28] Red knot populations are greatly affected by climate change since middle and high arctic habitats are necessary for breeding.

As arctic breeding grounds continue to warm, red knot body size has decreased, and less success for survival of birds born in warmer years is reported.

Even more significantly, their wintering areas in the tropics have become more stabilized, resulting in shorter bill birds (likely due to the fact that stable conditions breed greater ecological success and less variability within species).

[32] Indirect threats including horseshoe crab over harvesting and climate change greatly threaten red knot populations.

In New Jersey, state and local agencies are taking steps to protect these birds by limiting horseshoe crab harvesting and restricting beach access.

This followed a decade of intensive petitioning by environmental groups and a lawsuit against the Department of the Interior for alleged negligence in the protection of endangered species through failure to evaluate and list them.