| American black duck | |
|---|---|
 | |
| American black duck in flight | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | Anseriformes |
| Family: | Anatidae |
| Genus: | Anas |
| Species: | A. rubripes |
| Binomial name | |
| Anas rubripes Brewster, 1902 | |
 | |
Breeding Year-round Nonbreeding | |
| Synonyms | |
Anas obscuraGmelin, 1789 | |
TheAmerican black duck (Anas rubripes) is a largedabbling duck in the familyAnatidae. It was described byWilliam Brewster in 1902. It is the heaviest species in the genusAnas, weighing 720–1,640 g (1.6–3.6 lb) on average and measuring 54–59 cm (21–23 in) in length with an 88–95 cm (35–37 in)wingspan. It somewhat resembles the female and eclipse malemallard in coloration, but has a darkerplumage. The male and female are generally similar in appearance, but the male'sbill is yellow while the female's is dull green with dark marks on theupper mandible. It is native to easternNorth America. During the breeding season, it is usually found in coastal and freshwater wetlands fromSaskatchewan to the Atlantic inCanada and theGreat Lakes and theAdirondacks in the United States. It is a partiallymigratory species, mostly wintering in the east-central United States, especially in coastal areas.
It interbreeds regularly and extensively with the mallard, to which it is closely related. The female lays six to fourteen oval eggs, which have smooth shells and come in varied shades of white and buff green. Hatching takes 30 days on average.Incubation usually takes 25 to 26 days, with both sexes sharing duties, although the male usually defends the territory until the female reaches the middle of her incubation period. It takes about six weeks tofledge. Once the eggs hatch, the hen leads the brood to rearing areas with abundant invertebrates and vegetation.
The American black duck is considered to be a species of least concern by the International Union for Conservation of Nature (IUCN), although some populations of the species are in decline. It has long been valued as a game bird.Habitat loss due todrainage,global warming, filling of wetlands due tourbanization andrising sea levels are major reasons for the declining population of the American black duck. TheUnited States Fish and Wildlife Service has been purchasing and managing the habitat of this species in many areas to support the migratory stopover, wintering and breeding populations. The Atlantic Coast Joint Venture also protects habitat through restoration and land acquisition projects, mostly within their wintering and breeding areas.
AmericanornithologistWilliam Brewster described the American black duck asAnas obscura rubripes, for "red-legged black duck",[2] in his landmark article "An undescribed form of the black duck (Anas obscura)," inThe Auk in 1902, to distinguish between the two kinds of black ducks found inNew England. One of them was described as being comparatively small, with brownish legs and an olivaceous or dusky bill, and the other as being comparatively larger, with a lighter skin tone, bright red legs and a clear yellow bill.[2] The larger of the two was described asAnas obscura by the GermannaturalistJohann Friedrich Gmelin in 1789[1] in the 13th edition of theSystema Naturae, Part 2, and he based it on the "Dusky Duck" of Welsh naturalistThomas Pennant.[2] The current scientific name,Anas rubripes, is derived from Latin, withAnas meaning "duck" andrubripes coming fromruber, "red", andpes, "foot".[3]
Pennant, inArctic Zoology, Volume 2, described this duck as coming "from theprovince of New York" and having "a long and narrow dusky bill, tinged with blue: chin white: neck pale brown, streaked downwards with dusky lines."[2] In a typicalobscura, characteristics such as greenish black, olive green or dusky olive bill; olivaceous brown legs with at most one reddish tinge; the nape and pileum nearly uniformly dark; spotless chin and throat; fine linear and dusky markings on the neck and sides of the head, rather than blackish, do not vary with age or season.[2]
The American black duck weighs 720–1,640 g (1.59–3.62 lb) and measures 54–59 cm (21–23 in) in length with a 88–95 cm (35–37 in) wingspan.[4] This species has the highest mean body mass in the genusAnas, with a sample of 376 males averaging 1.4 kg (3.1 lb) and 176 females averaging 1.1 kg (2.4 lb), although its size is typically quite similar to that of the familiar mallard.[5][6] The American black duck somewhat resembles the female mallard in coloration, although the black duck's plumage is darker.[7] Males and females are generally similar in appearance, but the male'sbill is yellow while the female's is dull green with dark marks on theupper mandible,[8] which is occasionally flecked with black.[9][10] The head is brown, but is slightly lighter in tone than the darker brown body. The cheeks and throat are streaked brown, with a dark streak going through the crown and dark eye.[7] Thespeculum feathers areiridescent violet-blue with predominantly black margins.[8] The fleshy orange feet of the duck have dark webbing.[11]
Both male and female American black ducks produce similar calls to their close relative, the mallard, with the female producing a loud sequence ofquacks which falls in pitch.[12]
In flight, the white lining of theunderwings can be seen in contrast to the blackish underbody and upperside.[7][13] The purple speculum lacks white bands at the front and rear, and rarely has a white trailing edge. A dark crescent is visible on the median underwing primary coverts.[13]
Juveniles resemble adult females, but have broken narrow pale edges of underpart feathers, which give a slightly streaked rather than scalloped appearance, and the overall appearance is browner rather than uniformly blackish. Juvenile males have brownish-orange feet while juvenile females have brownish feet and a dusky greyish-green bill.[13]
The American black duck is endemic to eastern North America.[14] In Canada, the range extends from northeasternSaskatchewan toNewfoundland andLabrador.[7] In the United States, it is found in northernIllinois,Michigan,New Jersey,Ohio,Connecticut,Vermont,South Dakota, centralWest Virginia,Maine and on the Atlantic coast toNorth Carolina.[7][15]
The American black duck is a habitat generalist as it is associated with tidal marshes and present throughout the year in salt marshes from theGulf of Maine to coastalVirginia.[16] It usually prefers freshwater and coastal wetlands throughout northeastern America, including brackishmarshes,estuaries and edges ofbackwater ponds and rivers lined byspeckled alder.[7][15] It also inhabitsbeaver ponds, shallow lakes withsedges andreeds,bogs in openboreal andmixed hardwood forests, as well as forestedswamps.[15] Populations in Vermont have also been found in glacialkettle ponds surrounded by bog mats.[15] During winter, the American black duck mostly inhabits brackish marshes bordering bays, agricultural marshes, flooded timber, agricultural fields, estuaries andriverine areas.[15] Ducks usually take shelter from hunting and other disturbances by moving to brackish and fresh impoundments on conservation land.[4]
The American black duck is an omnivorous species[17] with a diverse diet.[18] It feeds by dabbling in shallow water and grazing on land.[17] Its plant diet primarily includes a wide variety of wetland grasses and sedges, and the seeds, stems, leaves and root stalks ofaquatic plants, such as eelgrass, pondweed andsmartweed.[7][8] Its animal diet includesmollusks,snails,amphipods,insects,mussels and smallfishes.[17][18]
During the breeding season, the diet of the American black duck consists of approximately 80% plant food and 20% animal food. The animal food diet increases to 85% during winter.[17] During nesting, the proportion of invertebrates increases.[8] Ducklings mostly eat water invertebrates for the first 12 days afterhatching, including aquatic snowbugs, snails,mayflies,dragonflies,beetles,flies,caddisflies andlarvae. After this, they shift to seeds and other plant food.[17]
The breeding habitat includes alkaline marshes, acid bogs, lakes, ponds, rivers, marshes, brackish marshes and the margins of estuaries and other aquatic environments in northern Saskatchewan,Manitoba, acrossOntario,Quebec and the Atlantic Canadian Provinces, plus the Great Lakes and theAdirondacks in the United States.[19] It is partially migratory, and many winter in the east-central United States, especially coastal areas; some remain year-round in the Great Lakes region.[20] This duck is a rare vagrant to Great Britain and Ireland, where over the years several birds have settled in and bred with the local mallard.[21] The resulting hybrid can present considerable identification difficulties.[21]
Nest sites are well-concealed on the ground, often in uplands.Egg clutches have six to fourteen oval eggs,[11] which have smooth shells and come in varied shades of white and buff green.[19] On average, they measure 59.4 mm (2.34 in) long, 43.2 mm (1.70 in) wide and weigh 56.6 g (0.125 lb).[19] Hatching takes 30 days on average.[11] Theincubation period varies,[19] but usually takes 25 to 26 days.[22] Both sexes share duties, although the male usually defends the territory until the female reaches the middle of her incubation period.[22] It takes about six weeks tofledge.[22] Once the eggs hatch, the hen leads the brood to rearing areas with abundant invertebrates and vegetation.[22]
The American black duck interbreeds regularly and extensively with the mallard, to which it is closely related.[23] Some authorities even consider the black duck to be asubspecies of the mallard instead of a separatespecies.Mank et al. argue that this is in error as the extent of hybridization alone is not a valid means to delimitateAnas species.[24]
It has been proposed that the American black duck and the mallard were formerly separated by habitat preference, with the American black duck's dark plumage giving it a selective advantage in shaded forest pools in eastern North America, and the mallard's lighter plumage giving it an advantage in the brighter, more openprairie and plains lakes.[25] According to this view, recent deforestation in the east and tree planting on the plains has broken down this habitat separation, leading to the high levels of hybridization now observed.[26] However, rates of past hybridization are unknown in this and most other avian hybrid zones, and it is merely presumed in the case of the American black duck that past hybridization rates were lower than those seen today. Also, many avian hybrid zones are known to be stable and longstanding despite the occurrence of extensive interbreeding.[23] The American black duck and the local mallard are now very hard to distinguish by means ofmicrosatellite comparisons, even if many specimens are sampled.[27] Contrary to this study's claims, the question of whether the Americanhaplotype is an original mallard lineage is far from resolved. Their statement, "Northern black ducks are now no more distinct from mallards than their southern conspecifics" only holds true in regard to the molecular markers tested.[24] As birds indistinguishable according to the set of microsatellite markers still can look different, there are other genetic differences that were simply not tested in the study.[24]
In captivity studies, it has been discovered that the hybrids followHaldane's rule, with hybrid females often dying before they reach sexual maturity, thereby supporting the case for the American black duck being a distinct species.[23][28]
The nest predators of the American black duck includeAmerican crows,gulls andraccoons, especially in tree nests.[17]Hawks andowls are also major predators of adults. Bullfrogs andsnapping turtles eat many ducklings.[17] Ducklings often catch diseases caused byprotozoan blood parasites transmitted by bites of insects such asblackflies.[17] They are also vulnerable to lead shot poisoning, known asplumbism, due to their bottom-foraging food habits.[17]
Since 1988, the American black duck has been rated asleast concern on theIUCN Red List of Endangered Species.[1] This is because the range of this species is extremely large, which is not near the threshold of vulnerable species.[1] In addition, the total population is large, and, although it is declining, it is not declining fast enough to make the species vulnerable.[1] It has long been valued as a game bird, being extremely wary and fast flying.[29] Habitat loss due to drainage, filling of wetlands due to urbanization, global warming and rising sea levels are major reasons for the declining population.[14] Some conservationists consider hybridization and competition with the mallard as an additional source of concern should this decline continue.[30][31] Hybridization itself is not a major problem;natural selection makes sure that the best-adapted individuals have the most offspring.[32] However, the reduced viability of female hybrids causes some broods to fail in the long run due to the death of the offspring before reproducing themselves.[33] While this is not a problem in the plentiful mallard, it might place an additional strain on the American black duck's population. Recent research conducted for theDelta Waterfowl Foundation suggests that hybrids are a result offorced copulations and not a normal pairing choice by black hens.[34]
The United States Fish and Wildlife Service has continued to purchase and manage habitat in many areas to support the migratory stopover, wintering and breeding populations of the American black duck.[14] In addition, theMontezuma National Wildlife Refuge has purchased and restored over 1,000 acres of wetlands to provide stopover habitat for over 10,000 American black ducks during fall migration.[14] Also, the Atlantic Coast Joint Venture has been protecting the habitat of the American black duck through habitat restoration and land acquisition projects, mostly within their wintering and breeding areas.[14] In 2003, aBoreal Forest Conservation Framework was adopted by conservation organizations, industries andFirst Nations to protect the Canadian boreal forests, including the American black duck's eastern Canadian breeding range.[14]
