The perching ducks and related gooselike forms are a diverse array of some fourteen species that are largely subtropical to tropical in occurrence. Although they vary in size from as little as about a half a pound in the "pygmy geese" (Nettapus) to more than twenty pounds in the spur-winged geese (Plectropterus), all possess some common features.*These include a tendency toward hole-nesting, especially in trees; sharp claws; associated perching abilities; and long tails that presumably increase braking effectiveness when landing in trees. Nearly all species exhibit extensive iridescent coloration in the body, especially on the upper wing surface; this coloration is often exhibited by females as well as males. As a result, this tribe includes some of the most beautifully arrayed species of the entire family, of which the North American wood duck is an excellent example, as is the closely related Asian mandarin duck (Aix galericulata). The wood duck is the only perching duck that is native to the United States or Canada, but inasmuch as Mexico must be regarded as a part of North America, the inclusion of the muscovy duck as a North American species is fully justified.
Perching ducks, together with all of the following groups of waterfowl included in this book, are representatives of the large anatid subfamily Anatinae. Unlike the whistling ducks, swans, or true geese, species of this subfamily have a tarsal scale pattern that has vertically aligned scutes (scutellate condition) above the base of the middle toe, and the sexes are usually quite different in voice, plumage, and sexual behavior. These sexual differences can be attributed to the weaker and less permanent pair bonds characteristics of true ducks, with a renewal of pair bonds typically occurring each year. As a result, pair-forming behavior tends to be more complex and elaborate in these species, as a dual reflection of the greater and more frequent competition for mates and the need for safeguards in reducing or avoiding mixed pairings between species during the rather hurried pair-forming period. In these species, the males typically assume the initiative in pair-forming activities, and thus they are usually more colorful, more aggressive, and have the more elaborate pair-forming behavior patterns. On the other hand, the females retain a subdued, often concealing plumage pattern, associated with their assumption of most or all incubation and brood-rearing responsibilities. As a result, humans usually find it easy to recognize the distinctively plumaged males of most species, while the females of related species are often so similar that even experienced observers may find it difficult to identify them with certainty.
Following the initiation of incubation, the males in this subfamily typically abandon the females and begin their postnuptial molt, during which they become flightless for a time and usually also acquire a more female-like body plumage. Thus, unlike the species in the subfamily Anserinae, typical ducks have two plumages, and thus two body molts, per year. In males this double molt is most apparent, since the "eclipse" plumage attained following the postnuptial molt is usually less colorful and often quite female-like.
Although in all the species which have so far been studied the female also has a comparable summer molt and plumage, in most cases this plumage is so similar to the winter plumage that separate descriptions are not necessary. In most cases the "eclipse" plumage of males is held for only a few months, presumably to allow the male to regain the more brilliant plumage associated with pair formation as early as possible. In some cases, however, this "nuptial" plumage is not regained until well into winter (e.g., ruddy duck, Baikal teal, blue-winged teal), so that "summer" and "winter" plumages may be more or less recognizable. The situation is further complicated in the oldsquaw, which has a third partial molt in the fall (affecting both sexes but most apparent in the male) and which is restricted to the scapular region. Except in such special cases, the two major plumages of the male are referred to in the species accounts as "nuptial" and "eclipse" plumages, while the "adult" plumage of females refers to both of the comparable breeding and nonbreeding plumages.
The 115 species of waterfowl that belong to the subfamily Anatinae are grouped into a number of tribes, most of which include one or more native North American species. The only major tribe of Anatinae that is not represented in this continent is the shelduck tribe Tadornini, which has representatives in both South America and Eurasia. It is true that there are some old records of Atlantic coast occurrences for the ruddy shelduck (Tadorna ferruginea) and the common shelduck (Tadorna tadorna), as well as a few more recent sight records (Audubon Field Notes, 16:73; American Birds, 26:842; 27 :41), but these are quite possibly the result of escapes from captivity.
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