Anseriformes (Ducks, Geese, Swans, and Screamers)
AnseriformesFamily: Ducks, Geese, and Swans
(Ducks, geese, swans, and screamers)
Number of families 2
Number of genera, species 41 genera; 147 species
Evolution and systematics
Although the flamingos (Phoenicopteriformes) have webbed feet, goose-like calls, and complex bill laminae indicating a close relationship to the Anseriformes, there is compelling evidence that waterfowl more directly evolved from a galliform ancestor allied to the guans and curassows (Cracidae) of the New World. This theory is supported by the existence of the Neotropical screamers (Anhimidae), which share a number of anatomical features with waterfowl, and even more strongly with the unique magpie goose (Anseranas semipalmata) of Australia and New Guinea. The latter exhibits so many peculiar features that set it apart from all other ducks, geese, and swans and link it with the screamers that Sibley and Monroe have allocated its own monotypic family, the Anseranatidae.
The ancestors of present day waterfowl probably began their evolution in tropical swamps prior to the Eocene age more than 50 million years ago. However, the earliest undoubted fossil anserine fragments come from the latter part of the Eocene epoch discovered in deposits in Colorado, Utah, and France. Although over 100 extinct species of waterfowl have been described, many remains are indistinguishable from those of present-day species.
As of 2001, it is generally recognized that the 147 species and 41 genera of the Anatidae fall into seven sub-families: magpie goose (Anseranatinae), whistling-ducks (Dendrocyginae), stiff-tails (Oxyurinae), swans and geese (Anserinae), shelducks (Tadorninae), dabbling ducks (Anatinae), and sea ducks (Merginae). An alternative systematic interpretation of the number of taxa recognized as species was proposed by Bradley Livezey in 1997; based on phylogenetic (cladistic) analysis of 157 morphological features, Livezey suggested that the number of species be raised to 175. However, limits between several of these forms remain ill-defined; particularly difficult to interpret are relationships between the various forms of torrent duck (Merganetta armata), common eider (Somateria mollissima), Canada goose (Branta canadensis), and brant (Branta bernicla).
Members of the Anatidae show considerable variation in size and structure from the smallest, the tropical pygmy-geese (Nettapus spp.) measuring only 12 in (31 cm) and weighing a meager 10 oz (269 g), to the immense trumpeter swan (Cygnus buccinator) which is 72 in (183 cm) in length and can weigh well over 38 lb (17 kg). Waterfowl have relatively large, compact bodies, long or prominent necks, relatively short, stout tarsi, and, unlike the magpie goose, full webbing between the three forward-pointing toes (the hind toe or hallux is insignificant). To keep their plumage in good waterproofed condition, they are endowed with a particularly large preen gland. They have a distinctive bill structure, with the lower mandible flat and the upper roughly cone-shaped tapering to a drop-like hard process, the "nail," at the tip. Waterfowl have horny lamellae along the interior of the bill near the cutting edges, which provide a sifting apparatus. Waterfowl cannot soar or glide to any extent but fly directly and rapidly with their necks outstretched. Only five surviving species are habitually flightless: three of the four species of steamer-ducks (Tachyeres spp.), and the closely-related Auckland Island teal (Anas aucklandica) and Campbell Island teal (Anas nesiotis), these include the southernmost of all duck species, and all have very restricted ranges. The Anatidae have a relatively broad, usually pointed, wing composed of 10–11 primaries. Their tails are flat with 12–24 feathers and usually quite concealed by the folded wing. The young are nidifugous, have a dense downy plumage, and, except in some of the Oxyurinae, are tended for a long time by one or both parents.
Plumage coloration varies from the unpatterned white of most of the swans, through the drab brown of many of the geese to the brightly patterned and colored nuptial plumage of drakes of many northern ducks. The inconspicuous plumage of most female ducks, as with many other primarily ground-nesting birds, helps camouflage them while incubating. This does not apply to the males, most of which take no part in incubation. On the contrary, their conspicuous colors effectively enhance their species-specific display movements involved in pair formation and the maintenance of the pair bond. A metallic luster is especially developed on the secondaries of many ducks in the form of a colorful speculum, the pattern of which varies between species and no doubt helps as a species-recognition signal when mixed flocks of ducks take to the air. What might appear to be bright patterning in such colorful species as the harlequin duck (Histrionicus histrionicus) and wood duck (Aix sponsa) can be surprisingly cryptic, breaking up the shape of the bird and providing remarkable concealment when amongst the light and shade of dappled water or wave-splashed rocks.
In the true geese and swans, the sexes are similar in plumage and there is a single annual molt. However, the ducks—with few exceptions—are strongly sexually dimorphic and molt their contour feathers twice a year. In both groups there are exceptions, with several duck species undergoing only a partial second molt, while others like the long-tailed duck (Clangula hyemalis) have four partial plumage changes in a year. As a rule, female ducks have an inconspicuous plumage all year round, in which dull browns and grays predominate. Males of most migratory duck species have a pre-basic molt into a drab female-like "eclipse" plumage soon after breeding and undergo a pre-alternate molt back into their bright nuptial colors in the late fall and early winter when pair formation commences for many species.
In all ducks, swans, and geese the flight feathers are molted only once a year and dropped simultaneously as a completion of pre-basic molt, rendering the bird incapable of flight for a short period. The exception to this is the magpie goose (now allocated its own monotypic family) which has a gradual molt and retains the ability to fly throughout. The time of shedding the flight feathers is usually related to the reproductive cycle. In ducks, where only the female incubates and tends the young, males shed the flight feathers earlier than females. The latter shed these feathers only when the young are half grown. In the swans, the sequence is reversed; females become flightless soon after breeding and the males later, at a time when females can already make use of their wings to some extent. During the slow development of the young, the parents molt one after the other so that one of them is always ready to guide and defend the young. The ability to fly is reached by the young and regained by the adult male in late summer, the family is then ready to begin the fall migration or move to other waters.
Waterfowl are extremely widespread in suitable habitats, being absent only from Antarctica and many of the oceanic islands. Some 50% of the 150 species breed in the northern hemisphere, with half of these executing long distance migrations. This enables them to take advantage of the rich food sources and nesting space available during the short boreal summers, but forces them to migrate to more temperate regions to avoid the inhospitable winters. Such birds have been responsible for establishing isolated populations on some oceanic islands. In some cases these have already evolved to acceptable species levels, for example Laysan duck (Anas laysanensis) and Hawaiian duck (A. wyvilliana) of the central Pacific evolved from a relatively recent mallard ancestor, and Eaton's pintail (A. eatoni) of the southern Indian Ocean from northern pintail (A. acuta). Many southern dabbling ducks are endemic to various islands, having presumably arisen from a common ancestor; most remarkable among these compose the eight species within the austral teal complex: Madagascar teal (A. bernieri), Andaman teal (A. albogularis), Sunda teal (A. gibberifrons), gray teal (A. gracilis), chestnut teal (A. castanea), brown teal (A. chlorotis), Auckland Island teal, and Campbell Island teal, the last four forming the more southerly "brown teal" group of species, and the first four the "gray teal" group. Of the many genera the most widespread is Anas (the dabbling ducks) which contains almost one third of all waterfowl species and is found worldwide. Surprisingly only one (the monotypic Lophodytes, hooded merganser) is endemic to north America and only two to Palearctic Eurasia (monotypic Marmaronetta [marbled duck] and Mergellus [smew]) but the two combined as the Holarctic region have 11 (of which five are monotypic but two are polytypic—Anser, with some 11 species and Branta, with at least five). In comparison, the southern hemisphere has no less than 10 genera endemic to the Neotropics (of which eight are monotypic), this preponderance of endemic genera to the southern hemisphere is supported by the eight monotypic Australasian genera and five monotypic genera in Africa. Some distribution patterns indicate that well-separated land-masses were once connected: the fulvous whistling-duck (Dendrocygna bicolor) and comb duck (Sarkidiornis melanotos) are found across South America, Africa, and Asia and the southern pochard (Netta erythrophthalma) and white-faced whistling-duck (Dendrocygna viduata) in both South America and Africa. Australasia has several very strange and presumably primitive species, apart from the peculiar magpie goose mentioned earlier, including the bizarre musk duck (Biziura lobata) and the weird freckled duck (Stictonetta naevosa), which is believed to be allied to the stiff-tails and is probably not far removed from the black-headed duck (Heteronetta atricapilla) of South America.
Waterfowl can be found by almost any sort of wetland, from Arctic shores to mountain streams, steppe lakes, steamy rainforests, rivers, and tidal estuaries. Even some of the most familiar species, the Canada goose and mallard for example, can be found in their various forms in some of the most remote places on earth, yet appear equally at home on park lakes in cities. However, the majority of species are extremely specialized and fussy about their environment. Some, such as the pink-headed duck (Rhodonessa caryophyllacea), have become extinct through being over-specialized, the latter almost certainly having been a bird of the now fragmented swamp
grasslands of north-eastern India. Arctic and subarctic coastal waters are the abode of the various species of eiders, which have a marine existence matched only by the ecologically similar but quite unrelated steamer-ducks in southern South America. Although spending most of their lives on the sea, the scoters (Melanitta spp.) move inland to nest by freshwater lakes and pools, as does the diminutive long-tailed duck. Fast-flowing mountain streams are favored by the torrent ducks of the Andes, and flowing water features as the habitat preference of the Brazilian merganser (Mergus octosetaceus), blue duck (Hymenolaimus malacorhynchos) of New Zealand, and Salvadori's duck (Salvadorina waigiuensis) of New Guinea. The beautiful harlequin duck seems to be equally at home by mountain rivers and rocky coasts. By complete contrast a number of species are closely associated with forests, at least in the breeding season, the goldeneyes (Bucephala spp.), most mergansers, maned duck (Chenonetta jubata), wood duck, and mandarin all nest in tree holes in temperate regions. Much of the interior of Australia is very arid; sudden rainfall can create huge temporary shallow lakes, which can trigger-off a sudden colonization following the arrival of large numbers of nomadic pink-eared ducks (Malacorhynchus membranaceus).
The majority of species are diurnal; in winter, flocks of most species of northern geese and swans feed in fields during the daytime, and fly to the safety of lakes or tidal flats at dusk, returning to their feeding areas at daybreak. However, if persecuted by man the reverse often happens. Most waterfowl are sociable when not nesting, with large winter gatherings of geese and swans being composed of many family units, which stay together through the first year of the young bird's life. Winter gatherings of waterfowl can be enormous, particularly notable being the thousands of snow geese (Chen caerulescens) that winter on farmland in the southern United States and the very dense concentrations, totaling some 300,000, of Baikal teal (Anas formosa) estimated wintering in South Korea. Most waterfowl are relatively solitary when nesting, although bar-headed geese (Anser indicus) form colonies composed of hundreds of nests by some Tibetan lakes and some colonies of black swans (Cygnus atratus) in Australia and New Zealand are numbered in their thousands.
Vocal communication between waterfowl is particularly important with the larger species that habitually flock prior to migration and keep together in family units during the ensuing winter. Males of the Anatidae have a sac-like dilatation, the bulla, at the bifurcation of the trachea. The whistling-ducks have a symmetrically placed bulla in both sexes. Looped trachea found in the magpie goose and the northern swans allow these birds to call with a far-carrying bugling quality. The mute swan (Cygnus olor) lacks such a resonating chamber; its trachea is straight and its voice is restricted to grunts, snorts, and hisses, which have given it its English name. Instead of vocalizations, mute swans keep in contact during flight by producing a "singing" wing sound, audible for more than a 109 yd (100 m). In geese, swans, and whistling-ducks, the voices of the two sexes are very similar; they do, however, substantially differ between the sexes of most species of ducks and sheldgeese. Males of most duck species are relatively quiet, some such as the marbled duck (Marmaronetta angustirostris) are virtually silent. The majority however do produce some
form of rasping or whistled sounds during courtship, which is more strongly developed in some species than in others: the far-carrying yodeling of spring long-tailed ducks is a very evocative sound and the whistled cries from drake wigeons are far from insignificant. Most female ducks utter a variety of abrupt grunts or quacks given for different occasions, and several Anas species have a well-known descending mocking cackle uttered by females when left alone by their partners. Shrill plaintive peeping cries are given by the small young of most species which help keep the family in contact while foraging.
Most geese and swans migrate in flocks along relatively narrow traditional fly-ways and have regular stop-over points en route where they can rest and feed in large numbers. Geese and swans form life-long pair bonds and migrate together with their youngsters of that year as family units; thus the young birds seemingly learn the route from older, experienced birds. Movements of many southern hemisphere waterfowl are closely linked with seasonal rainfall, those of Australia being particularly vulnerable to dramatic dispersive movements into the interior following rains and rapid range contractions towards the south-east following droughts. Several species also perform post-breeding molt-migrations, this may allow the birds to gather in large numbers for safety at a time when they are flightless and vulnerable and presumably produces a rich food source to sustain them at a difficult time.
Feeding ecology and diet
The Anatidae can be divided into several groups according to their methods of obtaining food. Many geese effectively graze on land, cropping grass and similar vegetation, and gleaning fallen seeds and grain. Swans, shelducks, and surface-feeding ducks either dabble at the surface of the water or mud, swinging their bills and sifting for small invertebrates and plant materials, or up-end by immersing their heads and necks with the rear of the body projecting upright above the water. In this way they can reach the bottom of shallow waters with their bills. Diving ducks also generally get their food from the bottom, but they can reach greater depths by diving completely below the surface. Most diving ducks rely on their feet for propulsion when underwater, but some, the long-tailed duck for example, use their wings as well. Perhaps the greatest maneuverability is achieved by the mergansers, who use only their feet to execute remarkably quick twists and turns in the pursuit of fish. In most waterfowl (the mergansers are the exception) there are rows of horny lamellae inside the mandibles which, together with the tongue, form a fine sifting apparatus. The tongue serves to push food items between the ridges of the bill in grazing birds, in dabbling ducks and swans it functions as a suction piston sucking the muddy water in through the tip of the beak and squeezing it out again at the base. This creates a characteristic chattering sound as food particles are filtered through the mandibles.
The young of most duck species are sexually mature when nine to 11 months old; thus they breed in the second summer of their life. Whistling-ducks generally breed when one year old, the true geese at three years, but the swans do not do so until they reach four to five years of age. The duration of the pair bond varies; in most species of ducks it ends with egg-laying, while in others, the males remain near the nest until the young hatch, and only then gather into continually growing flocks of males. In the swans and true geese, the males always guard the nest and take a share in leading the family after hatching. Thus a permanent pair bond results that is often so strong that a widowed bird will not pair up again for the rest of its life. Interestingly, such species show little or no plumage difference between the sexes, whereas the majority of ducks, which have short-term pair bonds lasting only through a single season, are strongly sexuallydimorphic.
Colorful male plumage, aided by vocalizations and display postures, is an important factor in selecting a mate of the correct species and in building pair bonds. This is particularly true of migratory northern species which have a relatively short period of time to complete the breeding process. In complete contrast, many southern, more resident species, even within the same genus, show little or no sexual dimorphism. The peculiar stiff-tails have a different strategy in which males select a territorial patch from which they vigorously display to attract females, the grotesque musk duck of Australia may mate with several females in a season, with the oldest and biggest males faring better than younger novices. Unlike many other families of birds courtship feeding is virtually unknown amongst waterfowl, and mutual preening is only normally undertaken by the whistling-ducks. Copulation almost always occurs on water, only the Hawaiian goose (Branta sandvicensis) and Cape Barren goose (Cereopsis novaehollandiae) like the unique magpie goose, copulate on land. Despite its aversion to water, the Hawaiian goose's precopulatory display includes head-dipping and washing movements, indicating that its ancestors formerly copulated on water. Males of all ducks, geese, and swans have a copulatory organ that is relatively long and curves to the left when evaginated from the cloaca during copulation. Such an erectile pseudo-penis is particularly long in the freckled duck and the stiff-tails, but is found in only a few other orders of birds.
Many species of waterfowl build their nests as close as possible to water; but a number of surface-feeding ducks, able to walk with relative ease, will nest over a mile away from water. Although most waterfowl nest on the ground, some of the shelducks prefer disused mammal burrows, while a number of other ducks nest in tree cavities. Mallards nest almost anywhere, sometimes even utilizing nests of gamebirds, or disused tree nests of other large birds such as crows. The nest is a hollow, lined with stems and leaves pulled in from around the nest site. Shortly before the last egg is laid, the females pluck nest-down from their underparts to complete the lining. The color of the down differs between species, hole-nesting ducks tend to have whitish down (presumably to help the incubating bird locate its clutch in darkness) while ground-nesting species have dark down as an aid to camouflage. The eggs of nearly all waterfowl are pale and unmarked, although they do become stained by the body of the incubating bird. An exception is the relatively large rich brown eggs laid by the peculiar white-backed duck (Thalassornis leuconotus) of Africa. The largest eggs, at 12.5 oz (355 g), are produced by the trumpeter swan, while those of the smallest ducks weigh little over 0.7 oz (20 g).
In general, only the females incubate; however, males of the magpie goose, black swan, and the whistling-ducks help with these duties. Sometimes a female will dump her eggs in strange nests before she has constructed her own. In this way, almost unbelievably large clutches may arise: as many as 87 eggs have been counted in a single redhead (Aythya americana) nest. The black-headed duck of South America always lays its eggs in strange nests and does not concern itself about its offspring. It prefers to lay into the nests of the rosybill (Netta peposaca) but often lays in the nests of quite unrelated birds, including those of gulls or birds of prey.
In most waterfowl species the female lays one egg per day until the clutch is complete; in the swans there is an interval of two to three days between each egg. Clutch size varies considerably, from as few as two in the musk duck, to as many as 22 in the mallard.
The young of the Anatidae are among the best developed precocial birds. They are covered with a dense downy feathering that becomes water repellent by becoming greased during contact with their mother's abdominal plumage. Only a few hours after the hatching the mother leads the brood to water. Leaving the nest site is difficult for the young of tree nesters because of the drop involved, but the ducklings are light, their bones are soft and pliable, allowing them to fall without harm. Ducklings feed independently from their first day; the adults, normally the female, merely warm and protect their offspring. In some swans, the care of the young includes carrying tired youngsters on the parent's back. Goose
families often return to their nests in the evening during the first few days. Ducks tend their youngsters until they are capable of flight (a period of some 40–70 days according to species). Geese and swans, as already mentioned, lead their young until the following spring.
As of 2001, 26 waterfowl species were listed as Endangered or Threatened. Three of the five species classed as Critically Endangered are probably already Extinct (crested shelduck, Tadorna cristata), pink-headed duck, and Madagascar pochard (Aythya innotata); however still surviving with tiny populations is the Brazilian merganser and Campbell Island teal. Four of the six Endangered waterfowl are also confined to islands, Madagascar teal and Meller's duck (Anas melleri) on Madagascar, brown teal (New Zealand) and Hawaiian duck, while the other two, swan goose (Anser cygnoides) and white-winged duck (Cairina scutulata), are found as fragmented populations in eastern Asia. A further 15 species are treated as Vulnerable, with another seven classed as Near Threatened, these are not yet considered rare enough to cause major concern.
Although hunting is a major factor, there is little doubt that habitat loss is the prime threat to populations of many species. Introduced predators have had a disastrous impact on the eco-systems of many island faunas in various parts of the world. Wetland drainage and methods of riverine management have been particularly harmful, with the building of dams making conditions unsuitable for species favoring faster flowing water. Many ports are situated near the mouths of large rivers, the accompanying spread of industrial development results in the pollution and loss of extensive areas of marshland. Although there is much international concern and legislation over oil pollution, accidents involving tankers do occur and their huge oil slicks spread death and destruction among waterbirds. As a counter-measure to these changes of the habitat, the management of seasonal flood plains and creation of freshwater marshes and shallow lakes as waterfowl refuges and reserves have been of much benefit to many species. The importance of protecting key wetlands cannot be overestimated, indeed an international conference on wetland conservation held in Iran in 1971, the Ramsar Convention, encouraged all countries to protect their key wetlands and submit them as sites of international importance; as of 2001, there were 126 countries that had signed up a total of 1,140 wetland sites. The work of many international conservation organizations, especially those of the Wildfowl and Wetlands Trust and Wetlands International, have done much to save rare species both by creating captive stock and in promoting wetland conservation.
Significance to humans
Humans have has historically had a close association with waterfowl as a food source. The greylag goose (Anser anser), swan goose, and mallard have long been known in domestication (for at least 4,000 years in the case of the greylag); all three were probably first domesticated in eastern Asia. Through selective breeding a number of local breeds, often bearing little resemblance to their wild ancestor, have been developed. By contrast the Muscovy duck came into domestication in South America. The mute swan was farmed in Europe during the middle ages. In Iceland colonial nesting common eiders have long been farmed for the excellence of the down that the females line their nests with. Swans have a number of myths and legends associated with them, these formed the basis for Tchaikovsky's ballet "Swan Lake." In Greek mythology, Zeus transformed into a swan to seduce the King of Sparta's beautiful wife, Leda (famed also as being the mother of Helen of Troy).
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Wildfowl and Wetlands Trust. Slimbridge, Glos GL2 7BT United Kingdom. Phone: +44 01453 891900.