# Comb duck

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The **comb duck** or **American comb duck** (***Sarkidiornis sylvicola***), is an unusual [duck](/source/Duck) found in tropical wetlands in continental [South America](/source/South_America) south to the [Paraguay River](/source/Paraguay_River) region in eastern [Paraguay](/source/Paraguay), southeastern [Brazil](/source/Brazil) and extreme northeastern [Argentina](/source/Argentina),[1] and as a [vagrant](/source/Vagrancy_(biology)) on [Trinidad](/source/Trinidad).

Most taxonomic authorities split this species and the [knob-billed duck](/source/Knob-billed_duck) from each other. The comb duck is generally smaller in size when compared to the knob-billed duck, and flanks are darker (black in males, medium grey in females).

## Taxonomy

Uncertainty surrounds the correct [systematic](/source/Systematics) placement of this species. Initially, it was placed in the [dabbling duck](/source/Dabbling_duck) [subfamily](/source/Subfamily) [Anatinae](/source/Anatinae). Later, it was assigned to the "[perching ducks](/source/Perching_duck)", a [paraphyletic](/source/Paraphyletic) assemblage of waterfowl most of which are intermediate between dabbling ducks and [shelducks](/source/Shelduck). As the "perching ducks" were split up, the comb duck was moved to the [Tadorninae](/source/Tadorninae) or shelduck [subfamily](/source/Subfamily).[2]

Analysis of [mtDNA](/source/MtDNA) [sequences](/source/DNA_sequence) of the [cytochrome *b*](/source/Cytochrome_b) and [NADH dehydrogenase](/source/NADH_dehydrogenase) [subunit](/source/Protein_subunit) 2 [genes](/source/Gene), however, suggests that it is a quite [basal](/source/Basal_(phylogenetics)) member of the Anatidae, vindicating the earliest placement, but its closest living relatives cannot be resolved to satisfaction without further study.[3]

## Description

It is one of the largest species of duck. Length can range from 56 to 76 cm (22 to 30 in), wingspan ranges from 116 to 145 cm (46 to 57 in) and weight from 1.03 to 2.9 kg (2.3 to 6.4 lb).[4][5][6] 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](/source/Iridescence) 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](/source/Beak).

If seen at a distance, immature comb ducks can also be mistaken for a [fulvous whistling duck](/source/Fulvous_whistling_duck) (*Dendrocygna bicolor*). However, knob-billed ducks in immature [plumage](/source/Plumage) are rarely seen without adults nearby and thus they are usually easily identified, too.[2]

## Ecology

It breeds in still [freshwater](/source/Freshwater) [swamps](/source/Swamp) and [lakes](/source/Lake) in the [tropics](/source/Tropics). It is largely resident, apart from dispersion in the wet season.[2]

This [duck](/source/Duck) feeds on vegetation by grazing or dabbling[2] and to a lesser extent on small fish, [invertebrates](/source/Invertebrate), 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.[7]

### Reproduction

Comb ducks nest mainly in tree holes,[2] also in tall grass. They line their nests with reeds, grass, or feathers, but not down.[7]

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.[7]

Females lay seven to 15[2] yellowish-white eggs. Several females may lay in a single "dump nest" containing up to 50 eggs.[7]

## References

1. Bencke, Glayson Ariel (2007): Avifauna atual do Rio Grande do Sul, Brasil: aspectos biogeográficos e distribucionais ["The Recent avifauna of Rio Grande do Sul: Biogeographical and distributional aspects"]. Talk held on 22 June 2007 at *Quaternário do RS: integrando conhecimento*, Canoas, Rio Grande do Sul, Brazil. [PDF abstract](http://www6.ufrgs.br/alpp/Resumos_Quaternario_RS.pdf)

1. Madge, Steve & Burn, Hilary (1987): *Wildfowl: an identification guide to the ducks, geese and swans of the world*. [Christopher Helm](/source/Helm_Identification_Guides), London. ISBN 0-7470-2201-1

1. Johnson, Kevin P. & Sorenson, Michael D. (1999). ["Phylogeny and biogeography of dabbling ducks (genus: *Anas*): a comparison of molecular and morphological evidence"](http://sora.unm.edu/sites/default/files/journals/auk/v116n03/p0792-p0805.pdf). *[Auk](/source/Auk_(journal))*. **116** (3): 792–805. [doi:10.2307/4089339](https://doi.org/10.2307/4089339). [JSTOR 4089339](https://www.jstor.org/stable/4089339)

1. Ogilvie & Young, *Wildfowl of the World*. New Holland Publishers (2004), ISBN 978-1-84330-328-2

1. Hilty, Steven L. (2002). [*Birds of Venezuela*](https://books.google.com/books?id=40mFwoALUFUC&pg=PA197). Princeton University Press. pp. 197–. ISBN 978-1-4008-3409-9.

1. [*Sarkidiornis melanotos* (Comb duck, Knob-billed duck)](http://www.biodiversityexplorer.org/birds/anatidae/sarkidiornis_melanotos.htm) [Archived](https://web.archive.org/web/20120414193101/http://www.biodiversityexplorer.org/birds/anatidae/sarkidiornis_melanotos.htm) 2012-04-14 at the Wayback Machine. biodiversityexplorer.org

1. [Honolulu Zoo](/source/Honolulu_Zoo) (2007): [Comb Duck](http://www.honoluluzoo.org/comb_duck.htm) [Archived](https://web.archive.org/web/20051125123239/http://www.honoluluzoo.org/comb_duck.htm) 2005-11-25 at the Wayback Machine. Retrieved 2007-06-08.

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Adapted from the Wikipedia article [Comb duck](https://en.wikipedia.org/wiki/Comb_duck) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Comb_duck?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.