Anatidae

Ducks, geese, and swans
Black-bellied whistling duck (Dendrocygna autumnalis)
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Aves Subclass: Neornithes Infraclass: Neognathae Superorder: Galloanserae Order: Anseriformes Family: Anatidae Vigors, 1825
Subfamilies
Anatinae Anserinae Aythyinae Dendrocygninae Merginae Oxyurinae Plectropterinae Stictonettinae Tadorninae Thalassorninae and see text

Anatidae is the biological family of medium to very large-sized birds in the order Anseriformes that includes the ducks, geese and swans, with members characterized by a broad, stocky body, short legs with partially webbed feet, long neck, and a somewhat flattened bill with horny lamellae and a hard "nail" at the tip. The family has a cosmopolitan distribution, occurring on all the world's continents except Antarctica and on most of the world's islands and island groups. The family contains around 146 species in 40 genera.

Members of Anatidae are adapted for swimming, floating on the water surface, and in some cases diving in at least shallow water. They are generally herbivorous, and are monogamous breeders. Almost half of the species undertake annual migrations.

Members of Anatidae play an important ecological role as part of food chains, serving as food for a wide diversity of predators, such as birds of prey (hawks, eagles, falcons), mammals (foxes), reptiles (crocodilians), and even larger fish. Particularly the young are vulnerable, but adults of various species also may be attacked in water (such as by the muskellunge), in flight (such as by the Peregrine falcon), and on the nest (foxes, hawks, eagles).

A few species have been domesticated for agriculture, and many others are hunted for food and recreation. They also provide important economic value as the source of feathers and down for bedspreads, pillows, and so forth. Beyond this, they offer an aesthetic value for people, whether seen swimming on a lake, flying overhead during migration, or even walking through the woods and across roads with young ones.

Five species have become extinct since 1600, and many more are threatened with extinction.

Description

The ducks, geese, and swans have a general body plan that is broad and elongated (Carboneras 1992). Diving species vary from this in being rounder. The wings are short and pointed, and supported by strong wing muscles that generate rapid beats in flight. They typically have long necks, although this varies in degree between species. The legs are short and strong and set far to the back of the body, more so in the more aquatic species. Combined with their body shape this can make some species awkward on land, but they are stronger walkers than other marine and water birds such as grebes or petrels.

The Anatidae are particularly distinguished from other groups of birds by the partially webbed feet and the somewhat flattened bills with horny lamellae. Most species have bills that are flattened to a greater or lesser extent and the bills of all species contain serrated lamellae, which are miniature ridges, like the 'teeth of a comb'. They act as a filter when feeding for organisms or plant matter. The lamellae are particularly well defined in the filter-feeding species. (Caroneras 1992). The bills also contain a hard "nail" or process at the tip (Tellkamp 2004). Geese that feed by grazing have strong bills, a wide "nail" at the tip to grasp vegetation, and a stout and flat lamellae (Tellkamp 2004). Ducks that strain food particles have blade-like lamellae, which are tightly packed in filtering specialists (Tellkamp 2004). Mergansers, which feed on fish, have serrated, backward pointing, almost tooth-like lamellae and a very narrow bill (Tellkamp 2004).

Members of the Anatidae also are characterized by a large preen gland that is crowned by a tuft of feathers (Tellkamp 2004). Males of this family also share a large external penis (Tellkamp 2004); Anatidae is one of the few families of birds that possess a penis (McCracken 2000).

The Anatidae are mostly medium to large-sized birds. The smallest members of the family are the cotton pygmy goose (Nettapus coromandelianus) with reaches up only to 26.5 centimeters (10.5 inches) and 164 grams (5.8 oz) and the African pygmy goose (Nettapus auritus), which reaches 33 centimeters (13 inches) and 230 grams (0.51 pounds). On the other hand, the trumpeter swan (Cygnus buccinator) reaches a body length of 1.8 meters (71 inches) and 13.5 kilograms (30 pounds). Some mute swans (Cygnus olor) may reach 22.5 kilograms (49.6 pounds) (Tellkamp 2004).

The Anatidae can range from inconspicuous and dull to spectacularly colorful (Tellkamp 2004). Their feathers are excellent at shedding water due to special oils. Many of the ducks display sexual dimorphism, with the males being more brightly colored than the females (although the situation is reversed in species like the paradise shelduck). The swans, geese, and whistling-ducks lack sexually dimorphic plumage.

Behavior, diet, and reproduction

Anatids are vocal birds, producing a range of quacks, honks, squeaks, and trumpeting sounds, depending on species; the female often has a deeper voice than the male (Todd 1991).

Anatids are generally herbivorous as adults, feeding on various water-plants, although some species also eat fish, mollusks, or aquatic arthropods. In a number of species, the young include a high proportion of invertebrates in their diet, but become purely herbivorous as adults (Todd 1991).

Tellkamp (2004) notes that about half of the species (47.6 percent) in Anatidae are either completely or partially migratory and the majority of the remaining species wander over wide areas. Some geese fly as high as 10,000 meters (32,800 feet) during migrations.

The anatids are generally seasonal and monogamous breeders. The level of monogamy varies within the family, many of the smaller ducks only maintain the bond for a single season and find a new partner the following year, whereas the larger swans, geese, and some of the more territorial ducks maintain pair bonds over a number of years.

Most species of Anmatidae are adapted for copulation on the water only. They construct simple nests from whatever material is close to hand, often lining them with a layer of down plucked from the mother's breast. In most species, only the female incubates the eggs. The young are precocial, and are able to feed themselves from birth (Todd 1991). One aberrant species, the black-headed duck, is an obligate brood parasite, laying its eggs in the nests of gulls and coots. While this species never raises its own young, a number of other ducks will occasionally lay eggs in the nests of conspecifics (members of the same species) in addition to raising their own broods.

Relationship with humans

Humans have had a long relationship with ducks, geese, and swans; they are important economically and culturally to humans. Duck, eider (large seaducks in the genus Somateria), and goose feathers and down (fine feathers found under the tougher exterior feathers) have long been popular for bedspreads, pillows, sleeping bags, and coats. The members of this family also have long been used for food and for sports hunting. On the flip side, some anatids are damaging agricultural pests, and have acted as vectors for zoonosis such as avian influenza.

Several duck species have benefited from an association with people. However, since 1600, five species of duck have become extinct, largely due to anthropogenic activities, and subfossil remains have suggested that humans caused numerous extinctions in prehistory. Today many more are considered threatened. Most of the historic and prehistoric extinctions were insular species, these species were vulnerable due to small populations (often endemic to a single island), and island tameness. Evolving on islands that lacked predators these species lost anti-predator behaviors as well as the ability to fly, and were vulnerable to human hunting pressure and introduced species. Other extinctions and declines are attributable to overhunting, habitat loss, and modification, as well as hybridization with introduced ducks (for example the introduced ruddy duck swamping genetically the white-headed duck in Europe). Numerous governments, and conservation and hunting organizations have made considerable progress in protecting ducks and duck populations through habitat protection and creation, laws and protection, and captive breeding programs.

Systematics

While the status of the Anatidae as a family is straightforward, and there is little debate about which species properly belong to it, the relationships of the different tribes and subfamilies within it are poorly understood.

The systematics of the Anatinae remains in a state of flux. It had traditionally been divided into six subfamilies, but Livezey (1986) suggests that the Anatidae are better treated in nine subfamilies. This classification was popular in the late 1980s to 1990s (Madge and Burn 1987). But mtDNA sequence analyses (Sraml et al. 1996; Johnson and Sorenson 1999) indicate that, for example, the dabbling and diving ducks do not belong in the same subfamily. However, mtDNA is an unreliable source for phylogenetic information in many waterfowl (especially dabbling ducks) due to their ability to produce fertile hybrids (Carboneras 1992), in rare cases possibly even beyond the level of genus. Due to the small sample size of many molecular studies available to date, mtDNA results must be considered with caution.

This article presents ten subfamilies: Anserinae, Aythyinae, Dendrocygninae, Merginae, Oxyurinae, Plectropterinae, Stictonettinae, Tadorninae, and Thalassorninae. However, this is just one of several possible ways of organizing the many species within the Anatidae.

Terres and NAS (1991) suggested that the Anatidae may be considered to consist of just 3 subfamilies (ducks, geese, and swans, essentially), which contain the groups as presented here as tribes, with the swans separated as subfamily Cygninae, the goose subfamily Anserinae also containing the whistling ducks, and the Anatinae containing all other clades (Terres and NAS 1991).

Tellkamp (2004) lists seven subfamilies: Anseranatinae (magpie goose), Anserinae (geese and swans), Antinae (wood ducks, dabbling ducks, pochards), Dendrocygninae (whistling-ducks), Merginae (sea ducks), Oxyurinae (stiff-tailed ducks), and Tadorninae (shelducks) (Tellkamp 2004). However, the magpie-goose is no longer considered to be part of the Anatidae, but is placed in its own family Anseranatidae.)

Subfamilies and genera

• Subfamily: Dendrocygninae (One pantropical genus, of distinctive long-legged goose-like birds)
  • Dendrocygna, whistling ducks (9 living species)

• Subfamily: Thalassorninae (One genus in Africa, believed to be most closely related to the subfamily Dendrocygninae, though also showing convergent similarities to the subfamily Oxyurinae)
  • Thalassornis, white-backed duck
    

    Mute swan

• Subfamily: Anserinae, swans and geese (Three to seven extant genera with 25 to 30 living species, mainly cool temperate Northern Hemisphere but also some Southern Hemisphere species, with the swans in one genus [two genera in some treatments], and the geese in three genera [two genera in some treatments]. Some other species are sometimes placed herein, but seem somewhat more distinct [see below])
  • Cygnus, true swans (7 species, 4 sometimes separated in Olor)
  • Anser, gray geese (7 species)
  • Chen, white geese (3 species, sometimes included in Anser)
  • Branta, black geese (8 living species)

• Subfamily: Stictonettinae (One genus in Australia, formerly included in the Oxyurinae, but with anatomy suggesting a distinct ancient lineage perhaps closest to the Anserinae, especially the Cape Barren Goose)
  • Stictonetta, freckled duck

• Subfamily: Plectropterinae (One genus in Africa, formerly included in the "perching ducks," but closer to the Tadorninae)
  • Plectropterus, spur-winged goose

Male Common shelduck

• Subfamily: Tadorninae - shelducks and sheldgeese (This group of larger, often semi-terrestrial waterfowl can be seen as intermediate between Anserinae and Anatinae. The 1986 revision (Livezey 1986) has resulted in the inclusion of 10 extant genera with about two dozen living species [one probably extinct] in this subfamily, mostly from the Southern Hemisphere but a few in the Northern Hemisphere, but the affiliations of several presumed tadornine genera has later been questioned (Johnson and Sorenson 1999), and the group in the traditional lineup is likely to be paraphyletic)
  • Pachyanas, Chatham Island duck (prehistoric)
  • Tadorna, shelducks (7 species, one probably extinct) - possibly paraphletic
  • Salvadorina, Salvadori's teal
  • Centrornis, Madagascar sheldgoose (prehistoric, tentatively placed here)
  • Alopochen, Egyptian goose and Mascarene shelducks (1 living species, 2 extinct)
  • Neochen, Orinoco goose
  • Chloephaga, sheldgeese (5 species)
  • Hymenolaimus, blue duck
  • Merganetta, torrent duck

A female mallard duck

• Subfamily: Anatinae, dabbling ducks and moa-nalos (The dabbling duck group, of worldwide distribution, were previously restricted to just one or two genera, but had been extended (Livezey 1986) to include 8 extant genera and about 55 living species, including several genera formerly known as the "perching ducks"; mtDNA on the other hand shows that the genus Anas is over-lumped and casts doubt on the diving duck affiliations of several genera [see below]. The moa-nalos, of which 4 species in 3 genera are known to date, are a peculiar group of flightless, extinct Anatidae from the Hawaiian Islands. Gigantic in size and with massive bills, they were believed to be geese, but have been shown to be actually very closely related to mallards. They arose filling the ecological niche of turtles, ungulates, and other megaherbivores.)
  • Anas: wigeons, gadwalls, teals, pintails, mallards, shovelers, etc (40-50 living species, 3 extinct) - paraphyletic
  • Lophonetta, crested duck
  • Speculanas, bronze-winged duck
  • Amazonetta, Brazilian duck
  • Chelychelynechen, Turtle-jawed moa-nalo (prehistoric)
  • Thambetochen, Large-billed moa-nalos (2 species, prehistoric)
  • Ptaiochen, Small-billed moa-nalo (prehistoric)

• Subfamily: Aythyinae, diving ducks (Some 15 species of diving ducks, of worldwide distribution, in 2 to 4 genera; The 1986 morphological analysis (Livezey 1986) suggested that the probably extinct pink-headed duck of India, previously treated separately in Rhodonessa, should be placed in Netta, but this has been questioned (Collar et al. 2001). Furthermore, while morphologically close to dabbling ducks, the mtDNA data indicates that a treatment as a distinct subfamily is indeed correct, with the Tadorninae being actually closer to dabbling ducks than the diving ducks are (Johnson and Sorenson 1999).
  • Netta, red-crested Pochard and allies (4 species, one probably extinct)
  • Aythya, pochards, scaups, etc. (12 species)

• Subfamily: Merginae, eiders, scoters, sawbills, and other sea-ducks
  

  Common goldeneye couple, male on the right.
  (There are 9 extant genera and some 20 living species; most of this group occur in the Northern Hemisphere, but a few [mostly extinct] mergansers in the Southern Hemisphere)
  • Chendytes, diving-geese (prehistoric)
  • Polysticta, Steller's Eider
  • Somateria, eiders (3 species)
  • Histrionicus, harlequin duck (includes Ocyplonessa)
  • Camptorhynchus, labrador duck (extinct)
  • Melanitta, scoters (3 species)
  • Clangula, long-tailed duck (1 species)
  • Bucephala, goldeneyes (3 species)
  • Mergellus, smew
  • Lophodytes, hooded merganser
  • Mergus, mergansers (5 living species, one extinct).

• Subfamily: Oxyurinae, stiff-tail ducks (A small group of 3 to 4 genera, with 2 to 3 of them monotypic and with 7 to 8 living species)
  • Oxyura, stiff-tailed ducks (5 living species)
  • Nomonyx, masked duck
  • Biziura, musk ducks (1 living species, provisionally placed here)
  • Heteronetta, black-headed duck

• Unresolved
  

  The rare white-winged wood duck, a species of unclear affiliation.
  

  Wood duck Aix sponsa
  The largest degree of uncertainty concerns whether a number of genera are closer to the shelducks or to the dabbling ducks. See also the monotypic subfamilies above, and the "perching ducks"
  • Coscoroba, coscoroba swan - Anserinae or same subfamily as Cereopsis?
  • Cereopsis, Cape Barren goose - Anserinae, Tadorninae, or own subfamily?
  • Cnemiornis, New Zealand geese (prehistoric) - as Cereopsis
  • Malacorhynchus, pink-eared ducks (1 living species) - Tadorninae, Oxyurinae or Dendrocheninae?
  • Sarkidiornis, comb duck - Tadorninae or closer to dabbling ducks?
  • Tachyeres, steamer ducks (4 species) - Tadorninae or closer to dabbling ducks?
  • Cyanochen, blue-winged goose - Tadorninae or more distant clade?
  • Nettapus, pygmy geese (3 species) - Anatinae or part of Southern Hemisphere radiation?
  • Pteronetta, Hartlaub's duck - traditionally dabbling ducks, but may be closer to Cyanochen
  • Cairina, muscovy duck and white-winged wood duck (2 species) - traditionally dabbling ducks, but may be paraphyletic, with one species in Tadorninae and the other closer to diving ducks
  • Aix, mandarin duck and wood duck (2 species) - dabbling ducks or Tadorninae?
  • Callonetta, ringed teal - dabbling ducks or Tadorninae?
  • Chenonetta, maned duck (1 living species) - dabbling ducks or Tadorninae? Includes Euryanas
  • Marmaronetta, marbled duck - Formerly dabbling ducks; actually a diving duck or a distinct subfamily

Species known from bones only

From subfossil bones found on Kauaʻi (Hawaiian Islands), two enigmatic waterfowl are known (Burney et al. 2001). The living and assignable prehistoric avifauna of the archipelago contains as Anseriformes Branta geese and their descendants, and the moa-nalos as mentioned above. The following taxa, although certainly new species, cannot be assigned even to subfamily; that Kauaʻi is the oldest of the large Hawaiian Islands, meaning the species may have been evolving in isolation for nearly up to 10 mya (since the Late Miocene), does not help in determining their affinities:

• Long-legged "shelduck," Anatidae sp. et gen. indet.
• Small-eyed duck, Anatidae sp. et gen. indet.

Similarly, Wetmore's goose (Geochen rhuax) from the Big Island of Hawaiʻi, and a gigantic goose-like anatid from Oʻahu are known only from very incomplete and in the former case much damaged bone fragments. The former has been alleged to be a shelduck (Short 1970), but this was generally dismissed because of the damage to the material and biogeographic considerations. The long-legged Kauaʻi bird, however, hints at the possibility of a former tadornine presence on the archipelago.

Fossil Anatidae

The fossil record of anatids is extensive, but many prehistoric genera cannot be unequivocally assigned to present-day subfamilies for the reasons given above. Some (such as Eonessa) seem to belong to subfamilies which are completely extinct. For prehistoric species of extant genera, see the respective genus accounts.

Eonessinae - extinct ancient anatids

• Eonessa (Eocene of Utah, USA)

Dendrocheninae - a more advanced relative of the whistling-ducks or a more ancestral relative of stifftail ducks paralleling whistling-ducks; probably extinct but Malacorhynchus might belong here

• Mionetta (Late Oligocene - Middle Miocene of Central Europe) - includes "Anas" blanchardi, "A." consobrina, "A." natator, "Aythya" arvernensis
• Manuherikia (Bathans Early/Middle Miocene of Otago, New Zealand)
• Dendrochen (Early - Late? Miocene) - includes "Anas" integra, "A." oligocaena
• Dendrocheninae gen. et sp. indet. (Late Miocene of Argentina) - dendrochenine?

Anserinae

• Cygnavus (Early Oligocene of Kazakhstan - Early Miocene of Germany)
• Cygnopterus (Middle Oligocene of Belgium - Early Miocene of France) - sometimes included in Cygnavus
• Megalodytes (Middle Miocene of California, USA)
• "cf. Megalodytes" (Haraichi Middle Miocene of Annaka, Japan)
• Anserobranta (Late Miocene of Central Europe) - includes "Anas" robusta, validity doubtful
• Presbychen (Temblor Late Miocene of Sharktooth Hill, USA)
• Afrocygnus (Late Miocene - Early Pliocene of east-central Africa)
• Paracygnus (Kimball Late Pliocene of Nebraska, USA)
• Eremochen (Pliocene)

Tadorninae

• Miotadorna (Bathans Early/Middle Miocene of Otago, New Zealand)
• Tadorninae gen. et sp. indet. (Calvert Middle Miocene of Maryland, USA)
• Balcanas (Early Pliocene of Dorkovo, Bulgaria) - may be synonym of Tadorna or even Common Shelduck
• Anabernicula (Late Pliocene ?- Late Pleistocene of SW and W North America)
• Brantadorna (Middle Pleistocene of Vallecito Creek, USA)
• Nannonetta (Late Pleistocene of Peru)

Anatinae

• Sinanas (Middle Miocene)
• Wasonaka (Middle Pliocene)

Oxyurinae

• Tirarinetta (Pliocene of Australia)

incertae sedis

• Guguschia (Oligocene of Azerbaijan) - Anserinae?
• "Anas" luederitzensis (Kalahari Early Miocene of Lüderitzbucht, Namibia) - Anatinae?
• Dunstanetta (Bathans Early/Middle Miocene of Otago, New Zealand)
• Matanas (Bathans Early/Middle Miocene of Otago, New Zealand)
• Anatidae gen. et sp. indet. MNZ S42797 (Bathans Early/Middle Miocene of Otago, New Zealand)
• "Oxura" doksana (Early Miocene of Dolnice, Czechia)
• "Aythya" chauvirae (Middle Miocene of Sansan, France and Credinţa, Romania) - 2 species
• Anatidae gen. et sp. indet. (Middle Miocene of Nördlinger Ries, Germany) - tadornine?
• Anatidae gen. et sp. indet. (Sajóvölgyi Middle Miocene of Mátraszõlõs, Hungary (Gál et al. 1998-99)
• "Anas" meyerii (Middle Miocene of Öhningen, Germany)
• "Anas" velox (Middle - Late? Miocene of C Europe) - Anatinae? May include "A." meyerii
• "Anas" albae (Late Miocene of Polgárdi, Hungary) - formerly in Mergus, Merginae?
• "Anas" isarensis (Late Miocene of Aumeister, Germany) - Anatinae?
• ?Anser scaldii (Late Miocene of Antwerp, Belgium) - anserine or tadornine
• "Anas" eppelsheimensis (Early Pliocene of Eppelsheim, Germany) - Anatinae?
• Aldabranas (Late Pleistocene of Aldabra, Indian Ocean) - Tadorninae or Anatinae
• "Chenopis" nanus - at least 2 taxa, may be living species (Pleistocene of Australia)

Putative or disputed prehistoric anatids are:

• Romainvillia (Late Eocene/Early Oligocene) - Anseranatidae or Anatidae (own subfamily)
• Loxornis (Deseado Early Oligocene of Argentina)
• Paracygnopterus (Early Oligocene of Belgium and England)
• Limicorallus (Indricotherium Middle Oligocene of Chelkar-Teniz, Kazakhstan)
• Teleornis (Deseado Early Oligocene of Argentina)
• Chenornis (Early Miocene) - Anserinae or Phalacrocoracidae
• Paranyroca (Rosebud Early Miocene of Bennett County, USA) - distinct Anatinae subfamily or own family
• Eoneornis (Miocene of Argentina) - Anatinae? A nomen dubium
• Eutelornis (Miocene of Argentina) - Anatinae?

References

ISBN links support NWE through referral fees

• Burney, D. A., H. F. James, L. P. Burney, S. L. Olson, W. Kikuchi, W. L. Wagner, M. Burney, D. McCloskey, D. Kikuchi, F. V. Grady, R. Gage, and R. Nishek. 2001. Fossil evidence for a diverse biota from Kauaʻi and its transformation since human arrival. Ecological Monographs 71(4): 615-641.

• Carboneras, C. 1992. Family Anatidae (ducks, geese and swans). Pages 536-629 in J. del Hoyo, A. Elliott, and J. Sargatal, eds., Handbook of Birds of the World, Volume 1: Ostrich to Ducks. Barcelona: Lynx Edicions. ISBN 8487334105.

• Collar, N. J., A. V. Andreev, S. Chan, M. J. Crosby, S. Subramanya, and J. A. Tobias. (Eds.) 2001. Pink-headed duck. Pages 489-501 in Threatened Birds of Asia: The BirdLife International Red Data Book. BirdLife International. ISBN 0946888442.

• Gál, E., J. Hír, E. Kessler, and J. Kókay. 1998-1999. Középsõ-miocén õsmaradványok, a Mátraszõlõs, Rákóczi-kápolna alatti útbevágásból. I. A Mátraszõlõs 1. lelõhely. [Middle Miocene fossils from the sections at the Rákóczi chapel at Mátraszőlős. Locality Mátraszõlõs I.]. Folia Historico Naturalia Musei Matraensis 23: 33-78. [Hungarian with English abstract] Retrieved January 30, 2009.

• Johnson, K. P., and M. D. Sorenson. 1999. Phylogeny and biogeography of dabbling ducks (genus Anas): A comparison of molecular and morphological evidence. Auk 116(3): 792–805. Retrieved January 30, 2009.

• Livezey, B. C. 1986. A phylogenetic analysis of recent anseriform genera using morphological characters. Auk 103(4): 737-754. Retrieved January 30, 2009.

• Madge, S., and H. Burn. 1987. Wildfowl: An Identification Guide to the Ducks, Geese and Swans of the World. London: Christopher Helm. ISBN 0747022011.

• McCracken, K. G. 2000. The 20-cm spiny penis of the Argentine lake duck (Oxyura vittata). Auk 117(3): 820–825.

• Short, L. L. 1970. A new anseriform genus and species from the Nebraska Pliocene. Auk 87(3): 537-543. Retrieved January 30, 2009.

• Sraml, M., L. Christidis, S. Easteal, P. Horn, and C. Collet. 1996. Molecular relationships within Australasian waterfowl (Anseriformes). Australian Journal of Zoology 44(1): 47-58. Retrieved January 30, 2009.

• Steadman, D. W. 1999. The prehistory of vertebrates, especially birds, on Tinian, Aguiguan, and Rota, Northern Mariana Islands. Micronesica 31(2): 319-345.

• Tellkamp, M. P. 2004. Ducks, geese, and swans (Anatidae). In B. Grzimek, D. A. Thoney, N. Schlager, J. E. Trumpey, and M. Hutchins, Grzimek's Animal Life Encyclopedia. Detroit: Thomson-Gale. ISBN 0787657778.

• Terres, J. K., and National Audubon Society (NAS). 1991. The Audubon Society Encyclopedia of North American Birds. New York: Wings Books. Reprint of 1980 edition. ISBN 0517032880.

• Todd, F. S. 1991. Anatidae. Pages 81-87 in J. Forshaw, Encyclopaedia of Animals: Birds. London: Merehurst Press. ISBN 1853911860.

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