Red knot – Calidris canutus
is a medium-sized shorebird which breeds in tundra and the Arctic Cordillera in the far north of Canada, Europe, and Russia.
An adult red knot is the second largest Calidris sandpiper, measuring 23–26 cm (9.1–10.2 in) long with a 47–53 cm (19–21 in) wingspan. The body shape is typical for the genus, with a small head and eyes, a short neck and a slightly tapering bill that is no longer than its head. It has short dark legs and a medium thin dark bill. The winter, or basic, plumage becomes uniformly pale grey, and is similar between the sexes. The alternate, or breeding, plumage is mottled grey on top with a cinnamon face, throat and breast and light-coloured rear belly. The alternate plumage of females is similar to that of the male except it is slightly lighter and the eye-line is less distinct.

An adult red knot is the second largest Calidris sandpiper, measuring 23–26 cm (9.1–10.2 in) long with a 47–53 cm (19–21 in) wingspan. The body shape is typical for the genus, with a small head and eyes, a short neck and a slightly tapering bill that is no longer than its head. It has short dark legs and a medium thin dark bill. The winter, or basic, plumage becomes uniformly pale grey, and is similar between the sexes. The alternate, or breeding, plumage is mottled grey on top with a cinnamon face, throat and breast and light-coloured rear belly. The alternate plumage of females is similar to that of the male except it is slightly lighter and the eye-line is less distinct. Canutus, islandica and piersmai are the “darker” subspecies. Subspecies rogersi has a lighter belly than either roselaari or piersmai, and rufa is the lightest in overall plumage. The transition from alternate to basic plumages begins at the breeding site but is most pronounced during the southwards migration. The molt to alternate plumage begins just prior to the northwards migration to the breeding grounds, but is mostly during the migration period.

The large size, white wing bar and grey rump and tail make it easy to identify in flight. When feeding the short dark green legs give it a characteristic ‘low-slung’ appearance. 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. They breed in the moist tundra during June to August. The display song of the male is a fluty poor-me. The display includes circling high with quivering wing beats and tumbling to the ground with the wings held upward. Both sexes incubate the eggs, but the female leaves parental care to the male once the eggs have hatched.

Juvenile birds have distinctive submarginal lines and brown coerts during the first year. In the breeding season the males can be separated with difficulty (<80% accuracy in comparison to molecular methods) based on the more even shade of the red underparts that extend towards the rear of the belly. It sounds like this
Recording by Andrew Spencer from Xeno canto