Anser (genus)
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Order: Anseriformes
Family: Anatidae
Subfamily: Anserinae

[Photo] Greylag Goose Anser anser in St James Park, London, England. Date 2005-09-16. Author: Piotr Kuczynski (http://commons.wikimedia.org/wiki/User:Pkuczynski).

Copyright (C) 2005 Piotr Kuczynski Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".

The waterfowl genus Anser includes all grey geese and usually the white geese too. It belongs to the true geese and swan subfamily (Anserinae). The genus has a Holarctic distribution, with at least one species breeding in any open, wet habitats in the subarctic and cool temperate regions of the Northern Hemisphere in summer; some also breed further south into warm temperate regions. They mostly migrate south in winter, typically to regions in the temperate zone between the January 0 °C (32 °F) to 5 °C (41 °F) isotherms.

The genus contains ten living species, which span nearly the whole range of true goose shapes and sizes. The largest is the Greylag Goose at 2.5-4.1 kg (5.5-9 lb) weight, and the smallest is the Ross's Goose at 1.2-1.6 kg. All have legs and feet that are pink, or orange, and bills that are pink, orange, or black. All have white under- and upper-tail coverts, and several have some sort of white pattern on their heads. The neck and body are either grey or white. The wings are grey or white with black or blackish primaries, and also often black or blackish secondaries. The closely related black geese in the genus Branta differ in having black legs, and generally darker body plumage.

Systematics, taxonomy and evolution

Living species
Swan Goose Anser cygnoides - sometimes separated in Cygnopsis
(Taiga) Bean Goose Anser fabalis
- Tundra Bean Goose Anser (fabalis) serrirostris
Pink-footed Goose Anser brachyrhynchus
White-fronted Goose Anser albifrons
- Greenland White-fronted Goose Anser (albifrons) flavirostris
Lesser White-fronted Goose Anser erythropus
Greylag Goose Anser anser
Bar-headed Goose Anser indicus - sometimes separated in Eulabeia
Snow Goose Anser caerulescens - sometimes separated in Chen
Ross's Goose Anser rossii - sometimes separated in Chen
Emperor Goose Anser canagicus - sometimes separated in Chen or Philacte

Taxonomy
The white geese are sometimes separated as the genus Chen, with one of them sometimes split off in the genus Philacte. They cannot be distinguished anatomically, there is some evidence of a distinct lineage in evaluations of molecular data. While most ornithological works traditionally include Chen within Anser, the AOU and the IUCN are notable authorities which treat them as separate.

Some authorities also treat some subspecies as distinct species (notably Tundra Bean Goose) or as likely future species splits (notably Greenland White-fronted Goose).

Fossil record
Numerous fossil species have been allocated to this genus. As the true geese are near-impossible to assign osteologically to genus, this must be viewed with caution. It can be assumed with limited certainty that European fossils from known inland sites belong into Anser. As species related to the Canada Goose have been described from the Late Miocene onwards in North America too, sometimes from the same localities as the presumed grey geese, it casts serious doubt on the correct generic assignment of the supposed North American fossil geese. The Early Pliocene Branta howardae is one of the cases where doubts have been expressed about its generic assignment. Similarly, Heterochen = Anser pratensis seems to differ profoundly from other species of Anser and might be placed into a different genus; alternatively, it might have been a unique example of a grey goose adapted for perching in trees.

Anser atavus (Middle/Late Miocene of Bavaria, Germany) - formerly in Cygnus
Anser arenosus (Big Sandy Late Miocene of Wickieup, USA)
Anser arizonae (Big Sandy Late Miocene of Wickieup, USA)
Anser cygniformis (Late Miocene of Steinheim, Germany)
Anser oeningensis (Late Miocene of Oehningen, Switzerland)
Anser thraceiensis (Late Miocene/Early Pliocene of Trojanovo, Bulgaria)
Anser pratensis (Valentine Early Pliocene of Brown County, USA) - possibly separable in Heterochen
Anser pressus (Glenns Ferry Late Pliocene of Hagerman, USA) - formerly Chen pressa
Anser thompsoni (Pliocene of Nebraska)
Anser azerbaidzhanicus (Early? Pleistocene of Binagady, Azerbaijan)

The Maltese swan Cygnus equitum was occasionally placed into Anser, and Anser condoni is a synonym of Cygnus paloregonus.

Ecology
The species are largely herbivorous wetland species, capable of swimming very well with their webbed feet, though they also graze extensively on dry land. When feeding in water, they will often up-end to reach deep submerged plants.

Breeding pairs mate normally for life. The nests are usually close to lakes or other water, most commonly on small islands to afford protection from mammalian predators, but also in some species on cliffs or crags near to water, or (rarely) in holes in trees. The nest is thickly lined with very soft down, plucked by the female parent from its breast and belly. From three to eight eggs are laid, these hatching in 21-30 days. The young, known as goslings, are led by both parents to water to feed in lakes or on land close to the lake edge where they can retreat quickly if danger threatens; both parents will defend the goslings vigorously. The goslings fledge in 40-60 days. Young birds are sexually mature at two years old and occasionally breed then, but do not normally breed until they are 3-4 years old.

Body size, bill size, development rates, and migration strategy are all strongly correlated, with small species breeding in high Arctic regions and larger species in temperate regions; the small species also have smaller bills, and their young develop more quickly in the short Arctic summer. The small species also have the longest migrations, moving further south to warmer regions in winter, while the large species, better able to cope with snow and ice cover, move shorter distances. Some populations of the largest species Greylag Goose are resident, not migrating at all. The genus thus follows Bergmann's Rule based on their wintering adaptations, but not to their breeding adaptation.

Breeding pairs are commonly strongly territorial, particularly defending a feeding area for their goslings, but they may also form sizable breeding colonies, particularly in Pink-footed Goose. All the species are gregarious to a greater or lesser degree in winter, when they can form spectacular flocks (e.g. up to 800,000 Snow Geese at a single site) grazing on farmland crops and stubble.

Relationship with humans and conservation status
Two species in the genus are of major commercial importance, having been domesticated as poultry: European domesticated geese are derived from the Greylag Goose, and Chinese and some African domesticated geese are derived from the Swan Goose.

Most species are hunted to a greater or lesser extent; in some areas, some populations are endangered by over-hunting. Most notably, the Lesser White-fronted Goose is listed by IUCN as Vulnerable throughout its range, and due to overhunting and rampant habitat destruction, the population of the Swan Goose is on the verge of collapsing, leading to a listing as Endangered.

Other species have benefitted from reductions in hunting since the late 19th/early 20th centuries, with most species in western Europe and North America showing marked increases in response to protection. In some cases, this has led to conflicts with farming, when large flocks of geese graze crops in the winter.

http://en.wikipedia.org/wiki/Grey_goose

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