Anas is a genus of dabbling ducks. It includes mallards, wigeons, teals, pintails and shovelers in a number of subgenera. Some authorities prefer to elevate the subgenera to genus rank. Indeed, as the moa-nalos are very close to this clade and may have evolved later than some of these lineages, it is rather the absence of a thorough review than lack of necessity that this genus is rather over-lumped. The phylogeny of this genus is one of the most confounded ones of all living birds. Research is hampered by the fact the radiation of the two major groups of Anas ? the teals and mallard groups ? took place in a very short time and fairly recently, roughly in the mid-late Pleistocene. Furthermore, hybridization may have long played a major role in Anas evolution, with within-subgenus hybrids regularly and between-subgenus hybrids not infrequently being fully fertile. The relationships between species are much obscured by this fact, and mtDNA sequence data is of dubious value in resolving their relationships; on the other hand, nuclear DNA sequences evolve too slowly to resolve the phylogeny of the subgenus Anas for example. Some major clades can be discerned. For example, that the traditional subgenus Anas, the mallard group, forms a monophyletic (in the loose sense, i.e. non-holophyletic) group has never been seriously questioned by modern science and is as good as confirmed (but see below). On the other hand, the phylogeny of the teals is very confusing. For these reasons, the dabbling duck lineages more distantly related to mallard group (which includes the type species of Anas) than the wigeons should arguably be separated in their own genera. These would include the Baikal Teal, the Garganey, the spotted black-capped Punanetta group, and the shovelers and other blue-winged species. Whether the wigeons, which are very distinct in morphology and behavior, but much less so in mtDNA cytochrome b and NADH dehydrogenase subunit 2 sequences, should also be considered a distinct genus Mareca (including the Gadwall and Falcated Duck) is essentially the one remaining point of dispute as regards the question which taxa should remain in this genus and which ones should not.
Listen to the sound of Hawaiian Duck
Copyright remark: Most sounds derived from xeno-canto
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A significant population decline in the early 20th century was brought about by nest predation by rats, mongooses, domestic dogs and cats, introduced fish and birds, habitat loss for agriculture and urban development, and local hunting pressure. The indiscriminate hunting of migratory waterbirds in the late 1800s and early 1900s took a heavy toll on the species’s population. Presently, hybridisation with feral A. platyrhynchos, and therefore the danger of genetic introgression, is the primary threat to the species’s recovery. A. platyrhynchos was first imported to Hawaii in the late 1800s for ornamental ponds and farming, and in the 1950s and 1960s, hundreds were imported to stock hunting areas. On Oahu, Maui and Hawaii there are very few pure birds remaining with the probable exception (on the latter) of birds at high elevations on Kohala and Mauna Kea, where there are few feral A. platyrhynchos. Hybridisation appears to be beginning on Kauai, the species’s largest population and so far largely free of hybrids. Wetland loss and habitat modification by alien aquatic plants are also threats. Pigs, goats and other feral ungulates may degrade nesting habitat. On Oahu at least, artificial wetlands associated with sugarcane plantations and aquaculture have disappeared as these industries have declined on the island. Introduced predators (such as the small Asian mongoose Herpestes javanicus, rats, cats and dogs) are an additional factor. The species is also threatened by drought and human disturbance from recreation and tourism.