Oca delle nevi

Identificazione

The snow goose has two color plumage morphs, white (snow) or gray/blue (blue), thus the common description as "snows" and "blues". White-morph birds are white except for black wing tips, but blue-morph geese have bluish-gray plumage replacing the white except on the head, neck and tail tip. The immature blue phase is drab or slate-gray with little to no white on the head, neck, or belly. Both snow and blue phases have rose-red feet and legs, and pink bills with black tomia ("cutting edges"), giving them a black "grin patch". The colors are not as bright on the feet, legs, and bill of immature birds. The head can be stained rusty-brown from minerals in the soil where they feed. They are very vocal and can often be heard from more than a mile away.<p>White- and blue-morph birds interbreed and the offspring may be of either morph. These two colors of geese were once thought to be separate species; since they interbreed and are found together throughout their ranges, they are now considered two color phases of the same species. The color phases are genetically controlled. The dark phase results from a single dominant gene and the white phase is homozygous recessive. When choosing a mate, young birds will most often select a mate that resembles their parents' coloring. If the birds were hatched into a mixed pair, they will mate with either color phase.<p>The species is divided into two subspecies on the basis of size and geography. Size overlap has caused some to question the division. The smaller subspecies, the lesser snow goose (C. c. caerulescens), lives from central northern Canada to the Bering Straits area. The lesser snow goose stands 64 to 79 cm (25 to 31 in) tall and weighs 2.05 to 2.7 kg (4.5 to 6.0 lb). The larger subspecies, the greater snow goose (C. c. atlanticus), nests in northeastern Canada. It averages about 3.2 kg (7.1 lb) and 79 cm (31 in), but can weigh up to 4.5 kg (9.9 lb). The wingspan for both subspecies ranges from 135 to 165 cm (53 to 65 in).

Tassonomia

In 1750 the English naturalist George Edwards included an illustration and a description of the snow goose in the third volume of his A Natural History of Uncommon Birds. He used the English name "The blue-winged goose". Edwards based his hand-coloured etching on a preserved specimen that had been brought to London from the Hudson Bay area of Canada by James Isham. When in 1758 the Swedish naturalist Carl Linnaeus updated his Systema Naturae for the tenth edition, he placed the snow goose with the ducks and geese in the genus Anas. Linnaeus included a brief description, coined the binomial name Anas caerulescens and cited Edwards' work. The snow goose is now placed in the genus Anser that was introduced by the French zoologist Mathurin Jacques Brisson in 1760. The scientific name is from the Latin anser, "goose", and caerulescens, "bluish", derived from caeruleus, "dark blue". The snow goose is the sister species to Ross's goose (Anser rossii).<p>Two subspecies are recognised:<p>The greater snow goose is distinguished from the nominate form by being slightly larger. It nests farther north and east. The lesser snow goose can be found in two color phases, the normal white-colored animals and a dark gray-colored "blue" phase. The greater snow goose is rarely seen in a blue phase.

Nome scientifico

Anser caerulescens

Autorità

(Linnaeus, 1758)

Ordine

Anseriformi

Famiglia

Anatidae

Genere

Oca

Areale e habitat

Areale riproduttivo

NA : n

Areale non riproduttivo

s, w USA and n Mexico

Migrazione

Complete migrants

Comportamento

Outside of the nesting season, they usually feed in flocks. In winter, snow geese feed on left-over grain in fields. They migrate in large flocks, often visiting traditional stopover habitats in spectacular numbers. Snow geese frequently travel and feed alongside greater white-fronted geese; in contrast, the two tend to avoid travelling and feeding alongside Canada geese, which are often heavier birds.[citation needed]<p>The population of greater snow geese was in decline at the beginning of the 20th century, but has now recovered to sustainable levels. Snow geese in North America have increased to the point where the tundra breeding areas in the Arctic and the saltmarsh wintering grounds are both becoming severely degraded, and this affects other species using the same habitat.<p>Major nest predators include Arctic foxes and skuas. The biggest threat occurs during the first couple of weeks after the eggs are laid and then after hatching. The eggs and young chicks are vulnerable to these predators, but adults are generally safe. They have been seen nesting near snowy owl nests, which is likely a solution to predation. Their nesting success was much lower when snowy owls were absent, leading scientists to believe that the owls, since they are predatory, were capable of keeping competing predators away from the nests. A similar association as with the owls has been noted between geese and rough-legged hawks. Additional predators at the nest have reportedly included wolves, coyotes and all three North American bear species. Few predators regularly prey on snow geese outside of the nesting season, but bald eagles (as well as possibly golden eagles) will readily attack wintering geese.

Conservazione

Tendenza della popolazione

rising