Red Knot
Encyclopedia
The Red Knot, Calidris canutus (just Knot in Europe
), is a medium sized shorebird
which breeds in tundra
and the Arctic Cordillera
in the far north of Canada
, Europe
, and Russia
. It is a large member of the Calidris sandpipers, second only to the Great Knot
. Six subspecies
are recognised.
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 Eurasia
n populations winter in Africa
, Papua New Guinea
, Australia
and New Zealand
. This species forms enormous flocks when not breeding.
of his Systema Naturae
as Tringa canutus. One theory is that it gets its name and species epithet from King Canute, Knot being another form of Canute; the name would refer to the knot's foraging along the tide line and the story of Canute and the tide. Another etymology is that the name is onomatopoeic, based on the bird's grunting call note.
The Red Knot and the Great Knot
were originally the only two species placed in the genus
Calidris but many other species of sandpiper were subsequently added. 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).
There are six subspecies
, in order of size;
Studies based on mitochondrial sequence divergence and models of paleoclimatic changes
during the glacial cycles suggest that canutus is the most basal population, separating about 20,000 years ago (95% confidence interval: 60,000–4,000 years ago) with two distinct lineages of the American and Siberian breeders emerging about 12,000 years ago (with a 95% confidence interval: 45,000–3,500 years ago).
, then migrates to coasts around the world from 50° N to 58° S. The red knot has one of the longest migrations of any bird. Every year it travels more than 9,000 miles from the Arctic to the southern tip of South America. The exact migration routes and wintering grounds of individual subspecies are still somewhat uncertain. The nominate race C. c. canutus breeds in the Taymyr Peninsula
and possibly Yakutia and migrates to the Western Europe and then down to western and southern Africa
. C. c. rogersi breeds in the Chukchi Peninsula
in eastern Siberia, and winters in eastern Australia
and New Zealand
. Small and declining numbers of rogersi (but possibly of the later described piersmai) winter in the mudflat
s in the Gulf of Mannar
and on the eastern coast of India. The recently split race C. c. piersmai breeds in the New Siberian Islands
and winters in north-western Australia. C. c. roselaari breeds in Wrangel Island
in Siberia and north-western Alaska
, and it apparently winters in Florida
, Panama
and Venezuela
. C. c. rufa breeds in the Canadian low Arctic, and winters South America, and C. c. islandica breeds in the Canadian high Arctic as well as Greenland
, and winters in Western Europe.
Birds wintering in west Africa were found to restrict their daily foraging to a range of just 2–16 km2 of intertidal area and roosted a single site for several months. In temperate regions such as the Wadden Sea they have been found to change roost sites each week and their feeding range may be as much as 800 km2 during the course of a week.
. 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 coverts 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.
The weight varies with subspecies, but ranges between 100 and 200 g (45–91 oz). Red Knots can double their weight prior to migration. Like many migratory birds they also reduce the size of their digestive organs prior to migration. The extent of the atrophy is not as pronounced as species like the Bar-tailed Godwit
, probably because there are more opportunities to feed during migration for the Red Knot. Red Knots are also able to change the size of their digestive organs seasonally. The size of the gizzard
increases in thickness when feeding on harder foods on the wintering ground and decreases in size while feeding on softer foods in the breeding grounds. These changes can be very rapid, occurring in as little as six days.
s, arthropod
s, and larvae obtained by surface pecking, and on the wintering and migratory grounds they eat a variety of hard-shelled prey such as bivalves, gastropods and small crab
s that are ingested whole and crushed by a muscular stomach.
While feeding in mudflats during the winter and migration Red Knots are tactile feeders, probing for unseen prey in the mud. Their feeding techniques include the use of shallow probes into the mud while pacing along the shore. 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 a centimetre depth. The bivalved mollusc Macoma
is their preferred prey on European coasts, swallowing them whole and breaking them up in their gizzard. In Delaware Bay
, they feed in large numbers on the eggs of horseshoe crabs which spawn just as the birds arrive in mid-summer. 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. Unlike many tactile feeders their visual field
is not panoramic (allowing for an almost 360 degree field of view), as during the short breeding season they switch to being visual hunters of mobile, unconcealed prey, which are obtained by pecking. Pecking is also used to obtain some surface foods in the wintering and migratory feeding grounds, such as the eggs of horseshoe crab
s.
The Red Knot is territorial
and seasonally monogamous
; it is unknown if pairs remain together from season to season. Males and females breeding in Russia have been shown to exhibit site fidelity
towards their breeding locales from year to year, but there is no evidence as to whether they exhibit territorial fidelity. Males arrive before females after migration and begin defending territories. As soon as males arrive, they begin displaying, and aggressively defending their territory from other males.
The Red Knot nests on the ground, near water, and usually inland. The nest
is a shallow scrape lined with leaves, lichens and moss
. Males construct three to five nest scrapes in their territories prior to the arrival of the females. The female lays three or more usually four eggs, apparently laid over the course of six days. The eggs measure 43 x 30 mm (1.7 x 1.2 in) in size and are ground coloured, light olive to deep olive buff, with a slight gloss. Both parents incubate
the eggs, sharing the duties equally. The off duty parent forages in flocks with others of the same species. The incubation period lasting around 22 days. At early stages of incubation the adults are easily flushed from the nest by the presence of humans near the nest, and may not return for several hours after being flushed. However in later stages of incubation they will stay fast on the eggs. Hatching of the clutch is usually synchronised. The chicks are precocial
at hatching, covered in downy cryptic feathers. The chicks and the parents move away from the nest within a day of hatching and begin foraging with their parents. The female leaves before the young fledge while the males stay on. After the young have fledged, the male begins his migration south and the young make their first migration on their own.
. 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. The quality of food at migratory stopover sites is a critical factor in their migration strategy.
This is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) applies. This commits signatories to regulate the taking of listed species or their eggs, to establish protected areas to conserve habitats for the listed species, to regulate hunting and to monitor the populations of the birds concerned.
. It is hypothesized that more recently, the birds have become threatened as a result of commercial harvesting of horseshoe crabs in the Delaware Bay
which began in the early 1990s. Delaware Bay is a critical stopover point during spring migration; the birds refuel by eating the eggs laid by these crabs (with little else to eat in the Delaware Bay),. If horseshoe crab abundance in the Bay is reduced there may fewer eggs to feed on which could negatively affect knot survival.
In 2003, scientists projected that at its current rate of decline the American subspecies might become extinct as early as 2010. As of April 2011 the subspecies is still extant. Several environmental groups have petitioned the U.S. government to list the birds as endangered, but thus far their requests have not been granted. In New Jersey, state and local agencies are taking steps to protect these birds by limiting horseshoe crab harvesting and restricting beach access. In Delaware, a two-year ban on the harvesting of horseshoe crabs was enacted but struck down by a judge who cited insufficient evidence that the ban would help restore the Red Knot's numbers to justify the potential disruption to the fishing industry. Instead, a male only harvest has been permitted in recent years.
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
), is a medium sized shorebird
Wader
Waders, called shorebirds in North America , are members of the order Charadriiformes, excluding the more marine web-footed seabird groups. The latter are the skuas , gulls , terns , skimmers , and auks...
which breeds in tundra
Tundra
In physical geography, tundra is a biome where the tree growth is hindered by low temperatures and short growing seasons. The term tundra comes through Russian тундра from the Kildin Sami word tūndâr "uplands," "treeless mountain tract." There are three types of tundra: Arctic tundra, alpine...
and the Arctic Cordillera
Arctic Cordillera
The Arctic Cordillera is a vast, deeply dissected chain of mountain ranges extending along the northeastern flank of the Canadian Arctic Archipelago from Ellesmere Island to the northeasternmost part of the Labrador Peninsula in northern Labrador and northern Quebec, Canada...
in the far north of Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, and Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
. It is a large member of the Calidris sandpipers, second only to the Great Knot
Great Knot
The Great Knot, Calidris tenuirostris, is a small wader. It is the largest of the calidrid species.Their breeding habitat is tundra in northeast Siberia. They nest on the ground laying about four eggs in a ground scrape. They are strongly migratory wintering on coasts in southern Asia through to...
. Six subspecies
Subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...
are recognised.
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
Bird migration
Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. Sometimes, journeys are not termed "true migration" because they are irregular or in only one direction...
to coastal areas in Europe and South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, while the Eurasia
Eurasia
Eurasia is a continent or supercontinent comprising the traditional continents of Europe and Asia ; covering about 52,990,000 km2 or about 10.6% of the Earth's surface located primarily in the eastern and northern hemispheres...
n populations winter in Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
, Papua New Guinea
Papua New Guinea
Papua New Guinea , officially the Independent State of Papua New Guinea, is a country in Oceania, occupying the eastern half of the island of New Guinea and numerous offshore islands...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
and New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
. This species forms enormous flocks when not breeding.
Taxonomy, systematics and evolution
This Red Knot was first described by Linnaeus in the landmark 1758 tenth edition10th edition of Systema Naturae
The 10th edition of Systema Naturae was a book written by Carl Linnaeus and published in two volumes in 1758 and 1759, which marks the starting point of zoological nomenclature...
of his Systema Naturae
Systema Naturae
The book was one of the major works of the Swedish botanist, zoologist and physician Carolus Linnaeus. The first edition was published in 1735...
as Tringa canutus. One theory is that it gets its name and species epithet from King Canute, Knot being another form of Canute; the name would refer to the knot's foraging along the tide line and the story of Canute and the tide. Another etymology is that the name is onomatopoeic, based on the bird's grunting call note.
The Red Knot and the Great Knot
Great Knot
The Great Knot, Calidris tenuirostris, is a small wader. It is the largest of the calidrid species.Their breeding habitat is tundra in northeast Siberia. They nest on the ground laying about four eggs in a ground scrape. They are strongly migratory wintering on coasts in southern Asia through to...
were originally the only two species placed in the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Calidris but many other species of sandpiper were subsequently added. A 2004 study found that the genus was polyphyletic
Polyphyly
A polyphyletic group is one whose members' last common ancestor is not a member of the group.For example, the group consisting of warm-blooded animals is polyphyletic, because it contains both mammals and birds, but the most recent common ancestor of mammals and birds was cold-blooded...
, and that the closest relative of the two knot species is the Surfbird
Surfbird
The Surfbird is a small stocky wader in the family Scolopacidae. It is usually classified in a genus of its own, as Aphriza virgata, and was once considered to be allied to the turnstones, but more recent data suggests it is very close genetically to the Red and Great Knots and should be included...
(currently Aphriza virgata).
There are six subspecies
Subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...
, in order of size;
- C. c. roselaari Tomkovich, 1990 (largest)
- C. c. rufa (Wilson, 1813)
- C. c. canutus (Linnaeus, 1758)
- C. c. islandica (Linnaeus, 1767)
- C. c. rogersi (Mathews, 1913)
- C. c. piersmai Tomkovich, 2001 (smallest)
Studies based on mitochondrial sequence divergence and models of paleoclimatic changes
Paleoclimatology
Paleoclimatology is the study of changes in climate taken on the scale of the entire history of Earth. It uses a variety of proxy methods from the Earth and life sciences to obtain data previously preserved within rocks, sediments, ice sheets, tree rings, corals, shells and microfossils; it then...
during the glacial cycles suggest that canutus is the most basal population, separating about 20,000 years ago (95% confidence interval: 60,000–4,000 years ago) with two distinct lineages of the American and Siberian breeders emerging about 12,000 years ago (with a 95% confidence interval: 45,000–3,500 years ago).
Distribution and migration
In the breeding season the Red Knot has a circumpolar distribution in the high ArcticArctic
The Arctic is a region located at the northern-most part of the Earth. The Arctic consists of the Arctic Ocean and parts of Canada, Russia, Greenland, the United States, Norway, Sweden, Finland, and Iceland. The Arctic region consists of a vast, ice-covered ocean, surrounded by treeless permafrost...
, then migrates to coasts around the world from 50° N to 58° S. The red knot has one of the longest migrations of any bird. Every year it travels more than 9,000 miles from the Arctic to the southern tip of South America. The exact migration routes and wintering grounds of individual subspecies are still somewhat uncertain. The nominate race C. c. canutus breeds in the Taymyr Peninsula
Taymyr Peninsula
The Taymyr Peninsula is a peninsula in the Far North of Russia, in the Siberian Federal District, that forms the northernmost part of mainland Eurasia and Asia...
and possibly Yakutia and migrates to the Western Europe and then down to western and southern Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
. C. c. rogersi breeds in the Chukchi Peninsula
Chukchi Peninsula
The Chukchi Peninsula, Chukotka Peninsula or Chukotski Peninsula , at about 66° N 172° W, is the northeastern extremity of Asia. Its eastern end is at Cape Dezhnev near the village of Uelen. It is bordered by the Chukchi Sea to the north, the Bering Sea to the south, and the Bering Strait to the...
in eastern Siberia, and winters in eastern Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
and New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
. Small and declining numbers of rogersi (but possibly of the later described piersmai) winter in the mudflat
Mudflat
Mudflats or mud flats, also known as tidal flats, are coastal wetlands that form when mud is deposited by tides or rivers. They are found in sheltered areas such as bays, bayous, lagoons, and estuaries. Mudflats may be viewed geologically as exposed layers of bay mud, resulting from deposition of...
s in the Gulf of Mannar
Gulf of Mannar
The Gulf of Mannar is a large shallow bay forming part of the Laccadive Sea in the Indian Ocean. It lies between the southeastern tip of India and the west coast of Sri Lanka. A chain of low islands and reefs known as Adam's Bridge, also called Ramsethu, which includes Mannar Island, separates the...
and on the eastern coast of India. The recently split race C. c. piersmai breeds in the New Siberian Islands
New Siberian Islands
The New Siberian Islands are an archipelago, located to the North of the East Siberian coast between the Laptev Sea and the East Siberian Sea north of the Sakha Republic....
and winters in north-western Australia. C. c. roselaari breeds in Wrangel Island
Wrangel Island
Wrangel Island is an island in the Arctic Ocean, between the Chukchi Sea and East Siberian Sea. Wrangel Island lies astride the 180° meridian. The International Date Line is displaced eastwards at this latitude to avoid the island as well as the Chukchi Peninsula on the Russian mainland...
in Siberia and north-western Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
, and it apparently winters in Florida
Florida
Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...
, Panama
Panama
Panama , officially the Republic of Panama , is the southernmost country of Central America. Situated on the isthmus connecting North and South America, it is bordered by Costa Rica to the northwest, Colombia to the southeast, the Caribbean Sea to the north and the Pacific Ocean to the south. The...
and Venezuela
Venezuela
Venezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...
. C. c. rufa breeds in the Canadian low Arctic, and winters South America, and C. c. islandica breeds in the Canadian high Arctic as well as Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
, and winters in Western Europe.
Birds wintering in west Africa were found to restrict their daily foraging to a range of just 2–16 km2 of intertidal area and roosted a single site for several months. In temperate regions such as the Wadden Sea they have been found to change roost sites each week and their feeding range may be as much as 800 km2 during the course of a week.
Description and anatomy
An adult Red Knot is the second largest Calidris sandpiper, measuring 23–26 cm (9–10 in) long with a 47–53 cm (18.5–21 in) wingspanWingspan
The wingspan of an airplane or a bird, is the distance from one wingtip to the other wingtip. For example, the Boeing 777 has a wingspan of about ; and a Wandering Albatross caught in 1965 had a wingspan of , the official record for a living bird.The term wingspan, more technically extent, is...
. The body shape is typical for the genus, with a small head and eyes, a short neck and a slightly tapering bill
Beak
The beak, bill or rostrum is an external anatomical structure of birds which is used for eating and for grooming, manipulating objects, killing prey, fighting, probing for food, courtship and feeding young...
that is no longer than its head. It has short dark legs and a medium thin dark bill. The winter, or basic, plumage
Plumage
Plumage refers both to the layer of feathers that cover a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage vary between species and subspecies and can also vary between different age classes, sexes, and season. Within species there can also be a...
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
Avian incubation
Incubation refers to the process by which certain oviparous animals hatch their eggs, and to the development of the embryo within the egg. The most vital factor of incubation is the constant temperature required for its development over a specific period. Especially in domestic fowl, the act of...
the eggs, but the female leaves parental care to the male once the eggs have hatched.
Juvenile birds have distinctive submarginal lines and brown coverts 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.
The weight varies with subspecies, but ranges between 100 and 200 g (45–91 oz). Red Knots can double their weight prior to migration. Like many migratory birds they also reduce the size of their digestive organs prior to migration. The extent of the atrophy is not as pronounced as species like the Bar-tailed Godwit
Bar-tailed Godwit
The Bar-tailed Godwit is a large wader in the family Scolopacidae, which breeds on Arctic coasts and tundra mainly in the Old World, and winters on coasts in temperate and tropical regions of the Old World...
, probably because there are more opportunities to feed during migration for the Red Knot. Red Knots are also able to change the size of their digestive organs seasonally. The size of the gizzard
Gizzard
The gizzard, also referred to as the ventriculus, gastric mill, and gigerium, is an organ found in the digestive tract of some animals, including birds, reptiles, earthworms and some fish. This specialized stomach constructed of thick, muscular walls is used for grinding up food; often rocks are...
increases in thickness when feeding on harder foods on the wintering ground and decreases in size while feeding on softer foods in the breeding grounds. These changes can be very rapid, occurring in as little as six days.
Diet and feeding
On the breeding grounds, Knots eat mostly spiderSpider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...
s, arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s, and larvae obtained by surface pecking, and on the wintering and migratory grounds they eat a variety of hard-shelled prey such as bivalves, gastropods and small crab
Crab
True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...
s that are ingested whole and crushed by a muscular stomach.
While feeding in mudflats during the winter and migration Red Knots are tactile feeders, probing for unseen prey in the mud. Their feeding techniques include the use of shallow probes into the mud while pacing along the shore. 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 a centimetre depth. The bivalved mollusc Macoma
Macoma
Macoma is a large genus of saltwater clams, marine bivalve mollusks in the family Tellinidae, the tellins.- Species :According to the World Register of Marine Species , the following species are included as accepted names within the genus Macoma * Macoma balthica * Macoma biota Arruda & Domaneschi,...
is their preferred prey on European coasts, swallowing them whole and breaking them up in their gizzard. In Delaware Bay
Delaware Bay
Delaware Bay is a major estuary outlet of the Delaware River on the Northeast seaboard of the United States whose fresh water mixes for many miles with the waters of the Atlantic Ocean. It is in area. The bay is bordered by the State of New Jersey and the State of Delaware...
, they feed in large numbers on the eggs of horseshoe crabs which spawn just as the birds arrive in mid-summer. They are able to detect molluscs buried under wet sand from changes in the pressure of water that they sense using Herbst corpuscles
Corpuscles of Herbst
The Corpuscles of Herbst or Herbst corpuscles are a nerve-ending similar to the Pacinian corpuscle, in the mucous membrane of the tongue, in pits on the beak and in other parts of the bodies of birds...
in their bill. Unlike many tactile feeders their visual field
Visual field
The term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments", while 'field of view' "refers to the physical...
is not panoramic (allowing for an almost 360 degree field of view), as during the short breeding season they switch to being visual hunters of mobile, unconcealed prey, which are obtained by pecking. Pecking is also used to obtain some surface foods in the wintering and migratory feeding grounds, such as the eggs of horseshoe crab
Horseshoe crab
The Atlantic horseshoe crab, Limulus polyphemus, is a marine chelicerate arthropod. Despite its name, it is more closely related to spiders, ticks, and scorpions than to crabs. Horseshoe crabs are most commonly found in the Gulf of Mexico and along the northern Atlantic coast of North America...
s.
Breeding
Territory (animal)
In ethology the term territory refers to any sociographical area that an animal of a particular species consistently defends against conspecifics...
and seasonally monogamous
Monogamy
Monogamy /Gr. μονός+γάμος - one+marriage/ a form of marriage in which an individual has only one spouse at any one time. In current usage monogamy often refers to having one sexual partner irrespective of marriage or reproduction...
; it is unknown if pairs remain together from season to season. Males and females breeding in Russia have been shown to exhibit site fidelity
Philopatry
Broadly, philopatry is the behaviour of remaining in, or returning to, an individual's birthplace. More specifically, in ecology philopatry is the behaviour of elder offspring sharing the parental burden in the upbringing of their siblings, a classic example of kin selection...
towards their breeding locales from year to year, but there is no evidence as to whether they exhibit territorial fidelity. Males arrive before females after migration and begin defending territories. As soon as males arrive, they begin displaying, and aggressively defending their territory from other males.
The Red Knot nests on the ground, near water, and usually inland. The nest
Bird nest
A bird nest is the spot in which a bird lays and incubates its eggs and raises its young. Although the term popularly refers to a specific structure made by the bird itself—such as the grassy cup nest of the American Robin or Eurasian Blackbird, or the elaborately woven hanging nest of the...
is a shallow scrape lined with leaves, lichens and moss
Moss
Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...
. Males construct three to five nest scrapes in their territories prior to the arrival of the females. The female lays three or more usually four eggs, apparently laid over the course of six days. The eggs measure 43 x 30 mm (1.7 x 1.2 in) in size and are ground coloured, light olive to deep olive buff, with a slight gloss. Both parents incubate
Avian incubation
Incubation refers to the process by which certain oviparous animals hatch their eggs, and to the development of the embryo within the egg. The most vital factor of incubation is the constant temperature required for its development over a specific period. Especially in domestic fowl, the act of...
the eggs, sharing the duties equally. The off duty parent forages in flocks with others of the same species. The incubation period lasting around 22 days. At early stages of incubation the adults are easily flushed from the nest by the presence of humans near the nest, and may not return for several hours after being flushed. However in later stages of incubation they will stay fast on the eggs. Hatching of the clutch is usually synchronised. The chicks are precocial
Precocial
In biology, the term precocial refers to species in which the young are relatively mature and mobile from the moment of birth or hatching. The opposite developmental strategy is called "altricial," where the young are born or hatched helpless. Extremely precocial species may be called...
at hatching, covered in downy cryptic feathers. The chicks and the parents move away from the nest within a day of hatching and begin foraging with their parents. The female leaves before the young fledge while the males stay on. After the young have fledged, the male begins his migration south and the young make their first migration on their own.
Status
The Red Knot has an extensive range, estimated at 0.1–1.0 million square kilometres (0.04–0.38 million square miles), and a large population of about 1.1 million individuals. The species is not believed to approach the thresholds for the population decline criterion of the IUCN Red List (i.e., declining more than 30% in ten years or three generations), and is therefore evaluated as Least ConcernLeast Concern
Least Concern is an IUCN category assigned to extant taxon or lower taxa which have been evaluated but do not qualify for any other category. As such they do not qualify as threatened, Near Threatened, or Conservation Dependent...
. 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. The quality of food at migratory stopover sites is a critical factor in their migration strategy.
This is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) applies. This commits signatories to regulate the taking of listed species or their eggs, to establish protected areas to conserve habitats for the listed species, to regulate hunting and to monitor the populations of the birds concerned.
Threats to the American subspecies
Towards the end of the 19th century, large numbers of Red Knot were shot for food as they migrated through North AmericaNorth America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. It is hypothesized that more recently, the birds have become threatened as a result of commercial harvesting of horseshoe crabs in the Delaware Bay
Delaware River
The Delaware River is a major river on the Atlantic coast of the United States.A Dutch expedition led by Henry Hudson in 1609 first mapped the river. The river was christened the South River in the New Netherland colony that followed, in contrast to the North River, as the Hudson River was then...
which began in the early 1990s. Delaware Bay is a critical stopover point during spring migration; the birds refuel by eating the eggs laid by these crabs (with little else to eat in the Delaware Bay),. If horseshoe crab abundance in the Bay is reduced there may fewer eggs to feed on which could negatively affect knot survival.
In 2003, scientists projected that at its current rate of decline the American subspecies might become extinct as early as 2010. As of April 2011 the subspecies is still extant. Several environmental groups have petitioned the U.S. government to list the birds as endangered, but thus far their requests have not been granted. In New Jersey, state and local agencies are taking steps to protect these birds by limiting horseshoe crab harvesting and restricting beach access. In Delaware, a two-year ban on the harvesting of horseshoe crabs was enacted but struck down by a judge who cited insufficient evidence that the ban would help restore the Red Knot's numbers to justify the potential disruption to the fishing industry. Instead, a male only harvest has been permitted in recent years.
External links
- Calls of Red Knot (Xeno-Canto)
- Cornell Lab of Ornithology - Red Knot
- USGS - Red Knot Information
- South Dakota Birds - Red Knot Information and Photos
- eNature.com - Red Knot
- In Search of the Red Knot-New Jersey Fish and Game
- US Fish and Wildlife Service
- The Shorebird Project
- Red Knot An Imperiled Migratory Shorebird in New Jersey