Anas platyrhynchos

From AOS (2020): The Anas platyrhynchos complex includes 14 closely related species; A. platyrhynchos appears to be most closely related to the New World radiation, which includes A. diazi, A. fulvigula, A. rubripes, A. wyvilliana, and A. laysanensis, and to A. poecilorhyncha J. R. Forster 1781 [Indian Spot-billed Duck] and A. zonorhyncha Swinhoe, 1866 [Eastern Spot-billed Duck] in the Old World (Lavretsky et al. 2014a). In various older treatments, some or even all New World taxa were treated as conspecific under the name A. platyrhynchos (e.g., Johnsgard 1961, 1967). Anas rubripes and A. fulvigula hybridize frequently with A. platyrhynchos in an area of broad overlap, largely as a result of introductions and range expansions of the latter into the range of A. rubripes and A. fulvigula. These 3 forms differ somewhat behaviorally and tend to segregate as species (Brodsky and Weatherhead 1984, Brodsky et al. 1988, Hepp et al. 1988, Ford et al. 2017, Lavretsky et al. 2019b), but early genetic studies found them difficult to differentiate (Ankney et al. 1986, Ankney and Dennis 1988, Avise et al. 1990, McCracken et al. 2001, Lavretsky et al. 2014a, b). More recent genomic studies have found that they are genetically separable (Lavretsky et al. 2015, 2019a,b), with differences likely the result of selection and demographic processes (Kirby et al. 2004, Lavretsky et al. 2019b). Further, genetic evidence suggests that hybridization is not as widespread as previously believed (Ford et al. 2017), and that A. platyrhynchos and A. rubripes do not represent a hybrid swarm (Lavretsky et al. 2019b).

Anas diazi was formerly (e.g., AOU 1983, 1998) considered conspecific with A. platyrhynchos, although prior to this (until AOU 1973) the 2 were treated as separate species. Newly separated based on assortative mating in the narrow contact zone between these species (Bellrose 1976, Hubbard 1977, Brown 1985) and genomic data that indicate restricted gene flow between them (Lavretsky et al. 2015, 2019a) (AOS 2020).

BREEDS: Alaska, Mackenzie Delta, southern Keewatin, and Maine south to southern California, Mexico, Oklahoma, and Virginia. Has expanded range in eastern North America (especially in the north) in recent decades (see Heusmann 1991 for a detailed account of status in the Atlantic Flyway). WINTERS: southern Alaska and southern Canada to southern U.S., Mexico, Cuba, occasionally Hawaii (AOU 1983). Half or more of the Mississippi Flyway's 3.2 million mallards winter in the lower Mississippi Valley, from Cape Girardeau, Missouri, to the Gulf of Mexico. Also occurs in the Palearctic. Many semiferal populations exist. Availablity of grain allows wintering north of pre-settlement range; now rare in Central America.

The recent decline is not simply a reflection of reduced number of wet ponds for breeding (Johnson and Shaffer 1987).