The knob-billed duck (''Sarkidiornis melanotos'') or African comb duck is a type of

duck Duck is the common name for numerous species of waterfowl in the family (biology), family Anatidae. Ducks are generally smaller and shorter-necked than swans and goose, geese, which are members of the same family. Divided among several subfam ...

found along the tropical/sub-tropical wetlands and waterways of

Sub-Saharan Africa Sub-Saharan Africa is the area and regions of the continent of Africa that lie south of the Sahara. These include Central Africa, East Africa, Southern Africa, and West Africa. Geopolitically, in addition to the list of sovereign states and ...

and the island of

Madagascar Madagascar, officially the Republic of Madagascar, is an island country that includes the island of Madagascar and numerous smaller peripheral islands. Lying off the southeastern coast of Africa, it is the world's List of islands by area, f ...

, as well as most of

South Asia South Asia is the southern Subregion#Asia, subregion of Asia that is defined in both geographical and Ethnicity, ethnic-Culture, cultural terms. South Asia, with a population of 2.04 billion, contains a quarter (25%) of the world's populatio ...

and mainland

Indochina Mainland Southeast Asia (historically known as Indochina and the Indochinese Peninsula) is the continental portion of Southeast Asia. It lies east of the Indian subcontinent and south of Mainland China and is bordered by the Indian Ocean to th ...

. Most taxonomic authorities classify the knob-billed duck and the

comb duck The comb duck or American comb duck (''Sarkidiornis sylvicola''), is an unusual duck, found in tropical wetlands in continental South America south to the Paraguay River region in eastern Paraguay, southeastern Brazil and extreme northeastern Ar ...

separately. A misidentified species of

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

Mauritian comb duck was initially described from unrecognised remains of the Mauritius sheldgoose (''Alopochen mauritiana''); this was realised as early as 1897, but the printed case of

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 defend ...

can still, occasionally, be found in modern-day sources.

Taxonomy and systematics

Uncertainty surrounds the correct

systematic Systematic may refer to: Science * Short for systematic error * Systematic fault * Systematic bias, errors that are introduced by an inaccuracy inherent to the system Economy * Systematic trading, a way of defining trade goals, risk control ...

placement of this species. Initially, it was placed in the

dabbling duck The Anatinae are a subfamily of the family Anatidae (swans, geese and ducks). Its surviving members are the dabbling ducks, which feed mainly at the surface rather than by diving. The other members of the Anatinae are the extinct moa-nalo, a y ...

subfamily In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus. Standard nomenclature rules end botanical subfamily names with "-oideae", and zo ...

Anatinae The Anatinae are a subfamily of the family Anatidae (swans, geese and ducks). Its surviving members are the dabbling ducks, which feed mainly at the surface rather than by diving. The other members of the Anatinae are the extinct moa-nalo, a ...

. Later, it was assigned to the "

perching duck The term perching ducks is used colloquially to mean any species of ducks distinguished by their readiness to perch high in trees. Until the late 20th century, perching ducks meant ''Cairinini'', a tribe of ducks in the duck, goose and swan fam ...

s", a

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

assemblage of waterfowl most of which are intermediate between dabbling ducks and

shelduck The shelducks, most species of which are found in the genus ''Tadorna'' (except for the Radjah shelduck, which is now found in its own monotypic genus ''Radjah''), are a group of large birds in the Tadorninae subfamily of the Anatidae, the biolog ...

s. As the "perching ducks" were split up, the knob-billed duck was moved to the

Tadorninae The Tadornini is a biological tribe that includes the shelducks and sheldgeese, which is placed in subfamily Anatinae of family Anatinae, which includes the ducks and most duck-like waterfowl such as the geese and swans. It has been treated a ...

or shelduck

. Analysis of

mtDNA Mitochondrial DNA (mtDNA and mDNA) is the DNA located in the mitochondria organelles in a eukaryotic cell that converts chemical energy from food into adenosine triphosphate (ATP). Mitochondrial DNA is a small portion of the DNA contained in ...

sequence In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is cal ...

s of the cytochrome ''b'' and

NADH dehydrogenase NADH dehydrogenase is an enzyme that converts nicotinamide adenine dinucleotide (NAD) from its reduced form (NADH) to its oxidized form (NAD+). Members of the NADH dehydrogenase family and analogues are commonly systematically named using the f ...

subunit 2

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

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.

Description

This common species is unmistakable. It is one of the largest species of duck. Length can range from , wingspan ranges from and weight from .''Sarkidiornis melanotos'' (Comb duck, Knob-billed duck)
. biodiversityexplorer.org 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

iridescence Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear gradually to change colour as the angle of view or the angle of illumination changes. Iridescence is caused by wave interference of light in microstru ...

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. Knob-billed ducks are generally larger in size when compared to comb ducks, and flanks are usually lighter (light grey, in females sometimes whitish). Immature knob-billed ducks look like a large greyish female of the

cotton pygmy goose The cotton pygmy goose or cotton teal (''Nettapus coromandelianus'') is a small perching duck which breeds in Asia, Southeast Asia extending south and east to Queensland where they are sometimes called white-quilled pygmy goose. They are among t ...

(''Nettapus coromandelicus'') and may be difficult to tell apart if no other birds are around to compare size and

hue In color theory, hue is one of the properties (called color appearance parameters) of a color, defined in the CIECAM02 model as "the degree to which a stimulus can be described as similar to or different from stimuli that are described as ...

. However, knob-billed ducks in immature

plumage Plumage () is a layer of feathers that covers a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage differ between species and subspecies and may vary with age classes. Within species, there can b ...

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.

Ecology

It breeds in still

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...

swamp A swamp is a forested wetland.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p. Swamps are considered to be transition zones because both land and water play a role in ...

s and

lake A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from ...

s in the

tropics The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...

. It is largely resident, apart from dispersion in the wet season. This

feeds on vegetation by grazing or dabbling and to a lesser extent on small fish,

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

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 The International Union for Conservation of Nature (IUCN) is an international organization working in the field of nature conservation and sustainable use of natural resources. Founded in 1948, IUCN has become the global authority on the status ...

. It is one of the species to which the '' Agreement on the Conservation of African-Eurasian Migratory Waterbirds'' applies.

Reproduction

African birds breed during and after the rainy season and may not breed if the rain is scanty. Knob-billed ducks nest mainly in tree holes, also in tall grass. Males may have two mates at once or up to five in succession. They defend the females and young but not the nest sites. Females lay 7 to 15 yellowish-white eggs.

Gallery

File:Sarkidiornis melanotos - Royal Museum for Central Africa - DSC06799.JPG, Specimens from the Royal Museum for Central Africa (female, male) File:CombDuck DSC0264.jpg, knob-billed duck at edge of lake near Jodhpur, Rajasthan, India, feeding alongside feral pigeons. File:CombDuck.jpg, Flying knob-billed duck / comb duck at

Chilika Lake Chilika Lake is the largest brackish water lagoon with estuarine character that sprawls along the east coast of Indian sub-continent in Asia and second largest coastal lagoon in the world, covering an area of over . It is spread over the Puri, ...

, Odisha, India File:Comb duck.jpg, Arampannai, Tamilnadu, India File:Knob-billed duck (Sarkidiornis melanotos) female in flight Jojawar.jpg, female, Jojawar, India File:Male knob-billed duck in water.jpg, Male knob-billed duck at Austin Roberts Bird Sanctuary in Pretoria, South Africa File:Knob-billed Duck, Bharatpur.jpg, Knob-billed Duck (Sarkidiornis melanotos) alongside

Eurasian Coot The Eurasian coot (''Fulica atra''), also known as the common coot, or Australian coot, is a member of the rail and crake bird family, the Rallidae. It is found in Europe, Asia, Australia, New Zealand and parts of North Africa. It has a slaty-bla ...

((Fulica atra) in

Keoladeo National Park Keoladeo National Park, or Keoladeo Ghana National Park, is a national park in Rajasthan, India. The national park hosts thousands of native, resident and migratory birds, especially during the winter season, when many different species fly to t ...

Rajasthan Rajasthan (; Literal translation, lit. 'Land of Kings') is a States and union territories of India, state in northwestern India. It covers or 10.4 per cent of India's total geographical area. It is the List of states and union territories of ...

India India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...

File:Knob-billed Duck AaMSM9151.jpg, Male at

Jamnagar Jamnagar () is a city and the headquarters of Jamnagar district in the Indian state of Gujarat. The city lies just to the south of the Gulf of Kutch, some west of the state capital, Gandhinagar. The city was the capital of Nawanagar State, Na ...

File:Knob-billed Duck Male AMSM8872.jpg, Close-up Male

References

External links

Species text in The Atlas of Southern African Birds
{{Taxonbar, from=Q426968 Sarkidiornis Birds of Sub-Saharan Africa Birds of India Birds of Southeast Asia knob-billed duck Taxa named by Thomas Pennant