A medium-sized dabbling duck that breeds throughout the north-central United States and Prairie Provinces of Canada, the Gadwall winters in the southern United States and coastal Mexico, the largest concentrations occurring along the Gulf Coasts of Louisiana and Texas. This species is monogamous; greater than 90% of females are paired by November, 4 to 5 months before breeding. During winter, individuals spend most of the day feeding on leaves and stems of aquatic vegetation in mixed flocks with other waterfowl and American Coots (Fulica americana). Peak spring migration occurs in March, and most pairs arrive on the breeding grounds in early April.
Gadwalls nest in tall, emergent vegetation near water and prefer islands. Their breeding home range overlaps with that of other pairs, but males defend a “moving” territory around a female. Because it nests in dense cover and often on islands, the Gadwall has higher nest success rates than do other species of prairie-nesting ducks. Gadwall pair bonds dissolve during incubation, and males join molting flocks while females continue to incubate. After their eggs hatch, females lead precocial young to brood-rearing habitat, where ducklings feed on a diet of invertebrates. Young are independent by 10 weeks of age, and their wings molt in preparation for fall migration.
Habitat degradation and drought conditions during the 1960s, 1970s, and early 1980s led to declines in many populations of waterfowl in the United States. Gadwall numbers in the Great Plains region and Prairie Provinces of Canada, however, increased 129% to record levels in the decade 1986–1996, owing to improved wetland conditions. In addition, this species has expanded its range into the northwestern United States and eastern Canada, establishing breeding populations in previously unoccupied habitat.
This species is well studied, compared to most other members of its genus. Several researchers have identified key aspects of both breeding and wintering biology. Important studies of breeding biology include Duebbert 1966 , Dwyer Dwyer 1974 , Dwyer 1975 , Blohm 1979 , and Hines and Mitchell Hines and Mitchell 1983a , Hines and Mitchell 1983b . Studies on winter ecology have focused on behavior and resource use ( Paulus 1980 , LeSchack 1993 ).