Comb Duck
Encyclopedia
The Knob-billed Duck or Comb Duck, is an unusual, pan-tropical duck
, found in tropical wetlands in sub-Saharan
Africa
, Madagascar
and south Asia
from Pakistan
to Laos
and extreme southern China
. It also occurs in continental South America
south to the Paraguay River
region in eastern Paraguay
, southeastern Brazil
and the extreme northeast of Argentina
, and as a vagrant
on Trinidad
.
It is the only known species of the genus Sarkidiornis. The supposed extinct "Mauritian Comb Duck" is based on misidentified remains of the Mauritian Shelduck
(Alopochen mauritianus); this was realized as early as 1897 but the mistaken identity
can still occasionally be found in recent sources.
especially prominent on the secondaries (lower arm feathers). The male is much larger than the female, and has a large black knob on the bill
. Young birds are dull buff below and on the face and neck, with dull brown upperparts, top of the head and eyestripe.
Immature Knob-billed Ducks look like a large greyish female of the Cotton Pygmy Goose
(Nettapus coromandelicus) and may be difficult to tell apart if no other birds are around to compare size and hue
. If seen at a distance, they can also be mistaken for a Fulvous Whistling-duck (Dendrocygna bicolor) or a female Maned Duck (Chenonetta jubata). The former is more vividly colored, with yellowish and reddish brown hues; the latter has a largely dark brown head with white stripes above and below the eye. However, Knob-billed Ducks in immature plumage
are rarely seen without adults nearby and thus they are usually easily identified too.
The Knob-billed Duck is silent except for a low croak when flushed.
There are two easily-distinguished subspecies
., in fact, some taxonomists consider them to be distinct species:
Uncertainty surrounds the correct systematic
placement of this species. Initially, it was placed in the dabbling duck subfamily Anatinae
. Later, it was assigned to the "perching duck
s", a paraphyletic assemblage of waterfowl most of which are intermediate between dabbling ducks and shelduck
s. As the "perching ducks" were split up, the Knob-billed Duck was moved to the Tadorninae
or shelduck subfamily. In addition, Some taxonomists separate the two subspecies into distinct species.
Analysis of mtDNA sequence
s of the cytochrome b
and NADH dehydrogenase
subunit
2 gene
s, however, suggests that it is a quite basal
member of the Anatidae, vindicating the earliest placement. But its closest living relatives cannot be resolved to satisfaction without further study.
swamp
s and lake
s in the tropics
. It is largely resident, apart from dispersion in the wet season.
This duck
feeds on vegetation by grazing or dabbling and to a lesser extent on small fish, invertebrate
s, and seeds. It can become a problem to rice farmers. Knob-billed Ducks often perch in trees. They are typically seen in flocks, small in the wet season, up to 100 in the dry season. Sometimes they separate according to sex.
The Knob-billed Duck is declining in numbers locally, but due to its wide range it is not considered globally threatened by the IUCN. It is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds
applies.
Males may have two mates at once or up to five in succession. They defend the females and young but not the nest sites. Unmated males perch in trees and wait for opportunities to mate.
Females lay 7 to 15 yellowish-white eggs. Several females may lay in a single "dump nest" containing up to 50 eggs
.
Duck
Duck is the common name for a large number of species in the Anatidae family of birds, which also includes swans and geese. The ducks are divided among several subfamilies in the Anatidae family; they do not represent a monophyletic group but a form taxon, since swans and geese are not considered...
, found in tropical wetlands in sub-Saharan
Sahara
The Sahara is the world's second largest desert, after Antarctica. At over , it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean...
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...
, Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
and south Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
from Pakistan
Pakistan
Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan...
to Laos
Laos
Laos Lao: ສາທາລະນະລັດ ປະຊາທິປະໄຕ ປະຊາຊົນລາວ Sathalanalat Paxathipatai Paxaxon Lao, officially the Lao People's Democratic Republic, is a landlocked country in Southeast Asia, bordered by Burma and China to the northwest, Vietnam to the east, Cambodia to the south and Thailand to the west...
and extreme southern China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
. It also occurs in continental 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...
south to the Paraguay River
Paraguay River
The Paraguay River is a major river in south central South America, running through Brazil, Bolivia, Paraguay, and Argentina...
region in eastern Paraguay
Paraguay
Paraguay , officially the Republic of Paraguay , is a landlocked country in South America. It is bordered by Argentina to the south and southwest, Brazil to the east and northeast, and Bolivia to the northwest. Paraguay lies on both banks of the Paraguay River, which runs through the center of the...
, southeastern Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
and the extreme northeast of Argentina
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
, and as a vagrant
Vagrancy (biology)
Vagrancy is a phenomenon in biology whereby individual animals appear well outside their normal range; individual animals which exhibit vagrancy are known as vagrants. The term accidental is sometimes also used...
on Trinidad
Trinidad
Trinidad is the larger and more populous of the two major islands and numerous landforms which make up the island nation of Trinidad and Tobago. It is the southernmost island in the Caribbean and lies just off the northeastern coast of Venezuela. With an area of it is also the fifth largest in...
.
It is the only known species of the genus Sarkidiornis. The supposed extinct "Mauritian Comb Duck" is based on misidentified remains of the Mauritian Shelduck
Mauritian Shelduck
The Mauritian Shelduck is an extinct species of goose from Mauritius. It was a close relative of the Egyptian Goose. Known from one or two subfossil carpometacarpus bones and a few descriptions, this bird was about the size of a Brent Goose. Its appearance is unknown, except that its wings showed...
(Alopochen mauritianus); this was realized as early as 1897 but the mistaken identity
Mistaken identity
Mistaken identity is a defense in criminal law which claims the actual innocence of the criminal defendant, and attempts to undermine evidence of guilt by asserting that any eyewitness to the crime incorrectly thought that they saw the defendant, when in fact the person seen by the witness was...
can still occasionally be found in recent sources.
Description and systematics
This common species is unmistakable. It is one of the largest species of duck. Length can range from 56 to 76 cm (22 to 29.9 in), wingspan ranges from 116 to 145 cm (45.7 to 57.1 in) and weight from 1.03 to 2.9 kg (2.3 to 6.4 lb). Adults have a white head freckled with dark spots, and a pure white neck and underparts. The upperparts are glossy blue-black upperparts, with bluish and greenish iridescenceIridescence
Iridescence is generally known as the property of certain surfaces which appear to change color as the angle of view or the angle of illumination changes...
especially prominent on the secondaries (lower arm feathers). The male is much larger than the female, and has a large black knob on the 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...
. Young birds are dull buff below and on the face and neck, with dull brown upperparts, top of the head and eyestripe.
Immature Knob-billed Ducks look like a large greyish female of the Cotton Pygmy Goose
Cotton Pygmy Goose
The Cotton Pygmy Goose or the Cotton Teal, Nettapus coromandelianus is a small perching duck which breeds in Pakistan, India, Bangladesh, southeast Asia and south to northern Australia.-Description:...
(Nettapus coromandelicus) and may be difficult to tell apart if no other birds are around to compare size and hue
Hue
Hue is one of the main properties of a color, defined technically , as "the degree to which a stimulus can be describedas similar to or different from stimuli that are described as red, green, blue, and yellow,"...
. If seen at a distance, they can also be mistaken for a Fulvous Whistling-duck (Dendrocygna bicolor) or a female Maned Duck (Chenonetta jubata). The former is more vividly colored, with yellowish and reddish brown hues; the latter has a largely dark brown head with white stripes above and below the eye. However, Knob-billed Ducks in immature 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...
are rarely seen without adults nearby and thus they are usually easily identified too.
The Knob-billed Duck is silent except for a low croak when flushed.
There are two easily-distinguished 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 fact, some taxonomists consider them to be distinct species:
- Knob-billed Duck (Sarkidiornis melanotos melanotos) (Pennant, 1769) (also called Nakta in South Asia) or Old World Knob-billed Duck (Sarkidiornis melanotos melanotos) from the Old WorldOld WorldThe Old World consists of those parts of the world known to classical antiquity and the European Middle Ages. It is used in the context of, and contrast with, the "New World" ....
- Larger; flanks lighter (light grey, in females sometimes whitish)
- Comb Duck (Sarkidiornis melanotos sylvicola) (IheringHermann von IheringHermann von Ihering was a German-Brazilian zoologist. He was born at Kiel, Germany, and died at Gießen, Germany. He was the oldest son of Rudolf von Jhering.-Biography:...
& Ihering, 1907) from South AmericaSouth AmericaSouth 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...
- Comb Duck (Sarkidiornis melanotos sylvicola) (Ihering
- Smaller; flanks darker (black in males, medium grey in females).
Uncertainty surrounds the correct systematic
Systematics
Biological systematics is the study of the diversification of terrestrial life, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees...
placement of this species. Initially, it was placed in the dabbling duck subfamily Anatinae
Anatinae
The Anatinae is a subfamily of the family Anatidae . Its surviving members are the dabbling ducks, which feed mainly at the surface rather than by diving...
. Later, it was assigned to the "perching duck
Perching duck
The perching ducks were previously treated as a small group of ducks in the duck, goose and swan family Anatidae, grouped together on the basis of their readiness to perch high in trees...
s", a paraphyletic assemblage of waterfowl most of which are intermediate between dabbling ducks and shelduck
Shelduck
The shelducks, genus Tadorna, are a group of large birds in the Tadorninae subfamily of the Anatidae, the biological family that includes the ducks and most duck-like waterfowl such as the geese and swans....
s. As the "perching ducks" were split up, the Knob-billed Duck was moved to the Tadorninae
Tadorninae
The Tadorninae is the shelduck-sheldgoose subfamily of the Anatidae, the biological family that includes the ducks and most duck-like waterfowl such as the geese and swans....
or shelduck subfamily. In addition, Some taxonomists separate the two subspecies into distinct species.
Analysis of mtDNA sequence
DNA sequence
The sequence or primary structure of a nucleic acid is the composition of atoms that make up the nucleic acid and the chemical bonds that bond those atoms. Because nucleic acids, such as DNA and RNA, are unbranched polymers, this specification is equivalent to specifying the sequence of...
s of the cytochrome b
Cytochrome b
Cytochrome b/b6 is the main subunit of transmembrane cytochrome bc1 and b6f complexes. In addition, it commonly refers to a region of mtDNA used for population genetics and phylogenetics.- Function :...
and NADH dehydrogenase
NADH dehydrogenase
NADH dehydrogenase is an enzyme located in the inner mitochondrial membrane that catalyzes the transfer of electrons from NADH to coenzyme Q...
subunit
Protein subunit
In structural biology, a protein subunit or subunit protein is a single protein molecule that assembles with other protein molecules to form a protein complex: a multimeric or oligomeric protein. Many naturally occurring proteins and enzymes are multimeric...
2 gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s, however, suggests that it is a quite basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
member of the Anatidae, vindicating the earliest placement. But its closest living relatives cannot be resolved to satisfaction without further study.
Ecology
It breeds in still freshwaterFreshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
swamp
Swamp
A swamp is a wetland with some flooding of large areas of land by shallow bodies of water. A swamp generally has a large number of hammocks, or dry-land protrusions, covered by aquatic vegetation, or vegetation that tolerates periodical inundation. The two main types of swamp are "true" or swamp...
s and lake
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
s in the tropics
Tropics
The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately N and the Tropic of Capricorn in the southern hemisphere at S; these latitudes correspond to the axial tilt of the Earth...
. It is largely resident, apart from dispersion in the wet season.
This duck
Duck
Duck is the common name for a large number of species in the Anatidae family of birds, which also includes swans and geese. The ducks are divided among several subfamilies in the Anatidae family; they do not represent a monophyletic group but a form taxon, since swans and geese are not considered...
feeds on vegetation by grazing or dabbling and to a lesser extent on small fish, invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s, and seeds. It can become a problem to rice farmers. Knob-billed Ducks often perch in trees. They are typically seen in flocks, small in the wet season, up to 100 in the dry season. Sometimes they separate according to sex.
The Knob-billed Duck is declining in numbers locally, but due to its wide range it is not considered globally threatened by the IUCN. It is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds
AEWA
The Agreement on the Conservation of African-Eurasian Migratory Waterbirds is the largest of its kind developed so far under the Bonn Convention. It was concluded on 16 June 1995 at The Hague, the Netherlands and entered into force on 1 November 1999 after the required number of at least fourteen...
applies.
Reproduction
African birds breed during and after the rainy season and may not breed if the rain is scanty (Honolulu Zoo). Knob-billed Ducks nest mainly in tree holes, also in tall grass. They line their nests with reeds, grass, or feathers, but not down.Males may have two mates at once or up to five in succession. They defend the females and young but not the nest sites. Unmated males perch in trees and wait for opportunities to mate.
Females lay 7 to 15 yellowish-white eggs. Several females may lay in a single "dump nest" containing up to 50 eggs
Bird egg
Bird eggs are laid by females and incubated for a time that varies according to the species; a single young hatches from each egg. Average clutch sizes range from one to about 17...
.