Anatinae: Dabbling Ducks
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The Anatinae is a subfamily of the family Anatidae (swans, geese and ducks).
Its surviving members are the dabbling ducks, which feed mainly at the surface rather than by diving. The other members of the Anatinae are the extinct moa-nalos, a young but highly apomorphic lineage derived from the dabbling ducks.
There has been much debate about the dabbling ducks' systematical status and what ducks belong to the Anatinae. As understood here, the subfamily contains only the dabbling ducks and their close relatives, the extinct moa-nalos.
Alternatively, the Anatinae are considered to include most "ducks" and the dabbling ducks form a tribe Anatini within these. The classification as presented here more appropriately reflects the remaining uncertainty about the interrelationships of the major lineages of Anatidae (waterfowl).
Systematics
The dabbling duck group, of worldwide distribution, was delimited in a 1986 study to include 8 genera and some 50-60 living species.
However, Salvadori's Teal is almost certainly closely related to the Pink-eared Duck, and other genera are likewise of unresolved affiliation. The peculiar Marbled Duck, formerly tentatively assigned to the dabbling ducks, is actually a diving duck or even a distinct subfamily.
This group of ducks is so named because its members feed mainly on vegetable matter by upending on the water surface, or grazing, and only rarely dive. These are mostly gregarious ducks of freshwater or estuaries. These birds are strong fliers and northern species are highly migratory. Compared to other types of duck, their legs are placed more towards the centre of their bodies. They walk well on land, and some species feed terrestrially.
Puddle ducks generally feed on the surface of the water or feed on very shallow bottoms. They are not equipped to dive down several feet like their diving counterparts. The most predominant difference between puddle ducks and divers are the size of the feet. Puddle ducks feet are generally smaller because they do not need the extra propulsion to dive for their forage.
Another distinguishing characteristic of puddle ducks when compared to diving ducks is the way in which they take flight when spooked or are on the move. Puddle ducks spring straight up from the water and diving ducks need to gain momentum to take off, so they must run across the water a short distance to gain flight.
History of classification
Traditionally, most ducks were assigned to either the shelducks, the perching ducks, and the dabbling and diving ducks; the latter two were presumed to make up the Anatinae. However, the perching ducks turned out to be a paraphyletic assemblage of various tropical waterfowl that simply had happened to evolve the ability to perch well in their forested habitat. Several of these, such as the Brazilian Duck, were subsequently assigned to the Anatinae.
As for the diving ducks, mtDNA cytochrome b and NADH dehydrogenase subunit 2 sequence data indicates that they are fairly distant from the dabbling ducks, the morphological similarities being due to convergent evolution.
In addition, the genus Anas as traditionally defined is not monophyletic; several South American species belong to a distinct clade which would include the Tachyeres steamer-ducks, and other species such as the Baikal Teal should also be considered distinct. See the genus article for more on this subject.
Genera
The following genera are (with one exception) unequivocal dabbling ducks:
- Amazonetta - status not fully resolved, most likely a dabbling duck
- Anas - probably paraphyletic:
- Lophonetta - formerly Anas
- Speculanas - formerly Anas
The 3 known genera and 4 known species of moa-nalos are went extinct around 1000 AD. They formerly occurred on the Hawaiian Islands and were derived from dabbling ducks, possibly even a close ancestor of the mallard:
- Chelychelynechen
- Thambetochen
- Ptaiochen
Subfossil remains of a small, flightless dabbling duck have been recovered on Rota in the Mariana Islands. These cannot be assigned to a known genus, but probably are closest to Anas. A most bizarre ducklike fowl has been found on the Hawaiian island of Kauaʻi. Due to its unique apomorphies (it seems to have had small eyes high and far back on its head) the placement of this anatid is likewise unresolved; only dabbling ducks and true geese are with certainty known to have colonized the Hawaiian chain.
Frequently placed into the Anatinae are these genera, whose relationships must be considered uncertain at present:
- Aix
- The Wood Duck (Aix sponsa), and the Mandarin Duck (Aix galericulata).
- Cairina - may be paraphyletic, with one species in Tadorninae and the other closer to diving ducks
- Muscovy Duck, Cairina moschata and White-winged Wood Duck, Cairina scutulata
- Callonetta - Tadorninae?
- Chenonetta - Tadorninae?
- Pteronetta - may belong into a distinct clade with Cyanochen
- Nettapus - part of ancient Gondwanan lineage?
On the other hand, the following genera, usually considered to belong into the Tadorninae, may actually be dabbling ducks:
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