There are two species of Knot in the world - Red Knots and Great Knots. They are both members of the Scolopacidae family (Sandpipers and similar small wading birds) and the Calidris genus ('Calidris' being derived from the Greek word skalidris used by Aristotle to describe some grey-backed wading birds that he encountered¹). The birds get their common name from the species name of the Red Knot - Calidris canutus. Canutus refers to King Canute, or Knut - like Canute, who famously demonstrated that he could not hold back the waves, Knots work with the tides rather than against them (they feed at low tide when mudflats are exposed, and roost at high tide when the mudflats are flooded). Also there is some evidence that Canute would have eaten Knots on occasion during his reign in the 11th Century CE, and 'knut' resembles the sound of the birds' call. Thus the birds called Knots are not to be confused with the fastenings or mathematical objects called knots.

Red Knots

Red Knots are so named because their breast plumage turns russet red in the breeding season². In winter they have grey backs and white undersides speckled with grey³. The global population of Red Knots has been assessed to be in excess of two million, but the numbers have decreased substantially since 2012, so they are classed as 'Near Threatened' on the International Union for Conservation of Nature's Red List (IUCN). The oldest Red Knot known to the Royal Society for the Protection of Birds (RSPB) was 27 years old, but their average life expectancy is eight years.

There are six subspecies of Red Knot, with somewhat different habits. For example, members of the Calidris canutus islandica subspecies breed in Greenland and Canada, visit Iceland and spend the winter in Europe, while the Calidris canutus canutus subspecies breeds in Siberia, visits Europe and spends the winter in Africa. Calidris canutus roselaari is the subspecies with the largest population, and Calidris canutus piersmai is the smallest. Calidris canutus rogersi winters in Australasia, and Calidris canutus rufa winters in South America.

In their breeding grounds in the Arctic, Red Knots mainly eat insects, plus spiders, snails, grass and some seeds. In winter they stay in coastal areas and feed on shellfish, crabs and worms. Each year 2-3 eggs are laid and both parents incubate them. The eggs hatch in about three weeks. The chicks are able to feed independently and fly by the time they are three weeks old. Female Knots begin to migrate before the chicks can fly. At a midpoint of their journey south, adult Knots will moult, shedding their bright breeding plumage and growing into their subtle winter tones. For protection against predators during this time, the birds flock together in large numbers (tens of thousands of birds may gather and create 'murmurations' like starlings as they settle down at a roosting site).

Great Knots

The Great Knot Calidris tenuirostris is the largest member of the Calidris genus. In summer its plumage is dark grey, and in winter it has a white underside and a brown and white back. It is classed as 'Endangered' on the IUCN Red List - there are estimated to be fewer than 300,000 of these birds left in the wild, and their population has decreased substantially since 2006 due to loss of habitat. They are migratory birds that mainly spend winters in Australia and summers in Russia, and require places to feed en route. They eat food such as shellfish, crustaceans and worms in coastal mudflats during winter, and eat berries and pine nuts during the breeding season. Chicks eat insects and spiders.

The species name tenuirostris means slender-billed. Knot beaks are sophisticated organs - the tips are so sensitive they can detect food items hidden in sand, and the beaks are strong enough to plough through sand to unearth food. The nasal glands in the beak enable Knots to drink sea water and eat salt-rich seafood - the glands excrete the salt but retain the water and nutrients.