The gadwall was first described by Carl Linnaeus in his landmark 1758 10th edition of Systema Naturae. DNA studies have shown that it is a sister species with the falcated duck; the two are closely related to the three species of wigeons, and all of them have been assigned to the genus Mareca. There are two subspecies:
- M. s. strepera, the common gadwall, described by Linnaeus, is the nominate subspecies.
- M. s. couesi, Coues's gadwall, extinct c. 1874, was formerly found only on Teraina, a coral atoll in the Pacific Ocean.
The gadwall is 46–56 cm (18–22 in) long with a 78–90 cm (31–35 in) wingspan. The male is slightly larger than the female, weighing on average 990 g (35 oz) against her 850 g (30 oz). The breeding male is patterned grey, with a black rear end, light chestnut wings, and a brilliant white speculum, obvious in flight or at rest. In non-breeding (eclipse) plumage, the drake looks more like the female, but retains the male wing pattern, and is usually greyer above and has less orange on the bill.
The female is light brown, with plumage much like a female mallard. It can be distinguished from that species by the dark orange-edged bill, smaller size, the white speculum, and white belly. Both sexes go through two moults annually, following a juvenile moult.
The gadwall is a quieter duck, except during its courtship display. Females give a call similar to the quack of a female mallard but higher-pitched, transcribed as gag-ag-ag-ag. Males give a grunt, transcribed as mep, and a whistle.
The gadwall breeds in the northern areas of Europe and across the Palearctic, and central North America. In North America, its breeding range lies along the Saint Lawrence River, through the Great Lakes, Alberta, Saskatchewan, the Dakotas, south to Kansas, west to California, and along coastal Pacific Canada and southern coastal Alaska. The range of this bird appears to be expanding into eastern North America. This dabbling duck is strongly migratory, and winters farther south than its breeding range, from coastal Alaska, south into Central America, and east into Idaho, Kansas, Ohio, Virginia, and then south all the way into Central America.
In Great Britain, the gadwall is a scarce-breeding bird and winter visitor, though its population has increased in recent years. It is likely that its expansion was partly through introduction, mainly to England, and partly through colonization to Great Britain, with continental birds staying to breed in Scotland. It has been reported in the River Avon in Hampshire and Wiltshire. In Ireland a small breeding population has recently become established, centred on Wexford in the south and Lough Neagh in the north.
The gadwall is a bird of open wetlands, such as prairie or steppe lakes, wet grassland or marshes with dense fringing vegetation, and usually feeds by dabbling for plant food with head submerged. It nests on the ground, often some distance from water. It is not as gregarious as some dabbling ducks outside the breeding season and tends to form only small flocks. Gadwalls are monogamous and may start breeding after their first year. Pair formation begins during fall migration or on breeding grounds, but has also been reported to occur in August when males are still in eclipse plumage. Gadwalls are generally quiet, except during courtship. The male utters a mep call during a display known as the burp, where he raises his head pointing his bill towards a female. The grunt-whistle is similar to that of mallards, where the male rears his outstretched head with the bill dipped into water, displacing a stream of water droplets towards a nearby female as the bill is raises against the chest. During this display the male makes a loud whistle call followed by a low burp. 
Young birds feed on insects at first; adults also eat some molluscs and insects during the nesting season.
Currently, the gadwall is listed as least concern in the IUCN Red List of Threatened Species. The gadwall is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) applies. Populations have increased approximately 2.5% over the course of 49 years (from 1966 to 2010), and continue to grow. Gadwalls are one of the most hunted duck species (3rd to the mallard and green-winged teal), with 1.7 million shot each year.
Because of the efforts of the United States and Canadian groups Ducks Unlimited, Delta Waterfowl Foundation and other private conservation groups, the species continues to be sustainably hunted there.
Eggs, collection Museum Wiesbaden
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A. macula alarum rufa nigra alba.
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- "gadwall". Merriam-Webster. Retrieved 24 August 2011.
- Floyd, T. (2008). Smithsonian Field Guide to the Birds of North America. New York: HarperCollins.
- Madge, Steve; Burn, Hilary (1988). Wildfowl: An Identification Guide to the Ducks, Geese and Swans of the World. Christopher Helm. pp. 200–202. ISBN 0-7470-2201-1.
- Dunn, J.; Alderfer, J. (2006). National Geographic Field Guide to the Birds of North America (5th ed.).
- "Gadwall". Irish Birds. 9 (1): 68. 2010.
- Baldassarre, Guy A. (2014). Ducks, geese, and swans of North America (2 ed.). Baltimore. ISBN 978-1-4214-0751-7. OCLC 810772720.
- Bishop, K. David (1999). "Preliminary notes on some birds in Bhutan" (PDF). Forktail. 15: 87. Archived from the original (PDF) on 2015-06-12. Retrieved 2014-11-30.
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|Wikimedia Commons has media related to Mareca strepera.|
|Wikispecies has information related to Anas strepera.|
|Wikisource has the text of the 1911 Encyclopædia Britannica article Gadwall .|
- BirdLife species factsheet for Anas strepera
- RSPB Birds by Name Gadwall Page
- Gadwall Species Account – Cornell Lab of Ornithology
- Gadwall – Anas strepera – USGS Patuxent Bird Identification InfoCenter
- Gadwal at the Massachusetts Breeding Bird Atlas
- "Gadwall media". Internet Bird Collection.
- Gadwall photo gallery at VIREO (Drexel University)
- Feathers of gadwall (Anas strepera)