The Mexican Duck (December 2009)
Taxonomists broadly accept interspecific hybridization by waterfowl (Anseriformes) as normal and not threatening to their status as biological species. For example, resident Pacific Black Ducks (Grey Ducks) hybridize extensively with Mallards that were introduced in large numbers to New Zealand starting 100 years ago (Rhymer et al 1994, Williams and Basse 2006). They are not sister species, and no authorities consider them to be conspecific.
Taxonomic treatment of Mallards (Anas platyrhyncos) and related species in North America has a related but more difficult history. This species complex includes five monomorphic taxa (Hawaiian Duck A.wyvilliana, Laysan Teal A.laysanensis, Mottled Duck A. fulvigula, Mexican Duck A.diazi and Black Duck A.rubripes) all of which were treated by some authorities as subspecies of the dimorphic Mallard (Peters 1931; Johnsgard 1978; Sibley and Monroe 1990). The AOU elevated the Laysan and Hawaiian ducks from subspecies to species status in the Sixth edition of the Checklist. This move gave the Laysan and Hawaiian ducks equivalent status to Black Duck and Mottled Duck, which the AOU was consistently recognized as species despite regular and sometimes extensive hybridization. Large numbers of pen raised Mallards released as hunting stocks are responsible for much of the hybridization with American Black Ducks in southern Canada and the northeastern US (Johnsgard & DiSilvestro 1976). Mallard x Black F1 hybrids and backcrosses (which revert to parental phenotypes) are numerous in some regions, but pure populations of Black Ducks persist and differences in behavior trump hybridization in their taxonomy (AOU 1983, Morton 1998).
The history of Mexican Duck differs from the other species in the North American Mallard complex. The AOU treated the Mexican Duck as a species through 5 editions of the AOU Checklist, but reduced it to subspecies rank in the Sixth edition of the Checklist (1983). The explanation was that “Extensive hybridization in southeastern Arizona, southern New Mexico, and west-central Texas compels merger of the two groups, formerly recognized as distinct species (AOU 1998, p 69). The AOU continues this treatment through the latest Supplement to the 7th edition of the Checklist, based on Hubbard’s (1977) analysis of hybridization.
Wikipedia summarizes the history as follows:
Including the Mexican Duck in the Mallard is a relict from the usual practice of much of the mid-late 20th century, when all North American "mallardines" as well as the Hawaiian and Laysan Ducks were included in the Mallard proper as subspecies. This was based on the assumption that hybridization, producing fertile offsprings, is an indicator of lack of speciation.
McCracken et al. (2001) challenged the subspecies status of the Mexican Duck. They found that the Mexican Duck is the southwestern sister “species” of the Mottled Duck and in turn the American Black Duck, all members of a set of original and monomorphic “mallards” that speciated in North America before dimorphic “greenhead” Mallards expanded their range from Europe to North America. They are all closely related members of a recent allopatric radiation with no postzygotic barriers to gene exchange between them. However, they mate assortatively, and do not interbreed freely. McCracken et al (2001) therefore recommended that “Mottled ducks and Mexican ducks be designated as species so that the nomenclature is consistent with phylogeny.”
The IOC World Bird List (2.3) follows this recommendation and restores Mexican Duck to the Master List.
A.O.U. 1983. Check-list of North American Birds. Sixth Edition
A.O.U. 1998. Check-list of North American Birds. Sixth Edition
Hubbard, JH. 1977. The biological and taxonomic status of the Mexican Duck. Bull. New Mexico Dept. Game Fish 16.
Johnsgard, PA. 1978. Ducks, geese, and swans of the world. Univ. Nebraska Press, Lincoln NB
Johnsgard, P. A. & DiSilvestro, R. 1976. Seventy-five years of changes in mallard–black duck ratios in eastern North America. Am. Birds 30: 905–908.
McCracken, KG., WP Johnson, & FH Sheldon. 2001. Molecular population genetics, phylogeography, and conservation biology of the mottled duck (Anas fulvigula). Conservation Genetics 2: 87-102.
Morton, ES. 1998. Pairing in mallards and American black ducks: a new view on population decline in American black ducks. Animal Conservation 1: 239–244.
Peters JL 1931. Check-list of the birds of the world, vol 1. Museum of Comparative Zoology, Cambridge MA
Rhymer JM, MJ Williams and MJ Braun. 1994. MtDNA gene flow between Grey Ducks and Mallards in New Zealand. Auk 111: 970-978. Bidirectional. Rhymer, JM and D Simberloff. 1996. Extinction by hybridization and introgression. Ann. Rev. Ecol. Syst. 27:83-109.
Sibley CG and BL Monroe. 1990. Distribution and taxonomy of the birds of the world. Yale University Press, New Haven CT
Williams, M. and B Basse. 2006. Indigenous gray ducks, Anas superciliosa, and introduced mallards, A. platyrhynchos, in New Zealand: processes and outcome of a deliberate encounter. Acta Zoologica Sinica 52(Supplement): 579–582.