Papers in the Biological Sciences
Date of this Version
2010
Abstract
The approximately twenty extant species of swans and true geese are, unlike the whistling ducks, primarily of temperate and arctic distribution, especially in the Northern Hemisphere. It is thus not surprising that continental North America may lay claim to at least nine breeding species, or nearly half.of the known total. Additionally, sufficient records of a tenth, the barnacle goose, are known as to warrant its inclusion in the book even though there is no indication that it nests in continental North America. Several additional Old World species of geese and swans have been reported one or more times in North America, but the likelihood of at least some of these being escapes from captivity seems so great that their inclusion seems unjustified. These species include the red-breasted goose (Branta ruficoIlis) , which has been collected in California at least five times and has also been seen in recent years in Texas, Pennsylvania, and Kansas, but is not known to nest nearer than central Siberia. The bean goose (Anser fabalis) has been reliably reported from Alaska (Byrd et aI., 1974), while the smaller pink-footed goose (A. f. brachyrhynchus) has been collected in Massachusetts (Bent, 1925) and seen in Delaware (Audubon Field Notes, 8:10, 9:235). Other Old World species that have been reported, such as the lesser white-fronted goose (Anser erythropus) and the bar-headed goose (Anser indicus), appear to have represented escapes from captivity, although a specimen of the former species was recently shot in Delaware (American Birds 27:597).
Geese and swans are generally large waterfowl that are almost entirely vegetarian in their diets. Swans forage predominantly in water, eating surface vegetation or tipping-up to reach underwater plants, but occasionally resort to eating terrestrial plants on shorelines or even in fields. Geese, however, forage both in water and on land, with some species such as brant foraging exclusively on aquatic life while others rely largely on terrestrial herbaceous plants. In most geese the cutting edges of the upper and lower mandibles are coarsely serrated in the manner of the pinking shears, providing an effective method of clipping off vegetation close to the ground. Like whistling ducks, swans and true geese have a reticulated tarsal pattern, lack iridescent or sexually dimorphic plumage patterns, and form strong, persistent pair bonds. Indeed, the fidelity of swan and goose pairs is legendary, although in actual fact this pairing behavior falls slightly short of their supposed perfect fidelity.
Although some authorities recognize a larger number of genera and species, recent investigators have generally recommended that only two or three swan genera be recognized (Coscoroba, Cygnus, and perhaps Olor) and that the genera of typical true geese be reduced to no more than three (Anser, Branta, and perhaps Nesochen). Likewise, species limits have been enlarged in recent years, so that the Old World and New World representatives of the arctic swans are now usually considered conspecific, the "blue goose" is generally recognized to be nothing more than a color phase of the snow goose, a single species of brant goose is recognized, and although a larger number of Canada goose races have recently been designated they are clearly part of an intergrading series of population complexes.
Comments
From Waterfowl of North America, Revised Edition (2010). Copyright © 2010 Paul A. Johnsgard.