Your first indication of their presence is the distant sound of baying hounds. As you look up, you see the sky flecked with tiny white moving shapes, which appear like snowflakes drifting lazily across the azure sky. This spectacle of Snow Geese in their large, high-flying migratory flocks described by the naturalist J. B. Grinnell is an ethereal one, the only one that many observers have of this species. Yet, this medium-sized goose is one of the most abundant species of waterfowl in the world, breeding in large, often dense colonies north of the tree line from extreme northeastern Russia (Wrangel Island), along the coast and islands of arctic and subarctic North America to northwestern Greenland. The species is composed of three fairly discrete regional breeding and wintering populations—western, midcontinent, and eastern—with birds from the same breeding population using similar staging and stopover areas each year while migrating south along roughly parallel lines of longitude to their wintering grounds. During migration and on the wintering grounds, they often congregate in very large flocks. The largest wintering populations occur in the central valleys of California (western), along the Gulf Coast of Louisiana and Texas (midcontinent), and along the Atlantic Coast from New Jersey to North Carolina (eastern), with smaller populations in the Pacific Northwest, inland in the central United States to Iowa and Nebraska, and in the southwestern United States and central highlands of Mexico.
The species is dimorphic, consisting of light-morph (white) and dark-morph (blue) Snow Geese. Color morphs are controlled by a single gene locus where the dark allele is incompletely dominant to the light. The distribution of color morphs is unequal in the population, with the maximum number of blue-morph geese occurring in midcontinent breeding and wintering areas. The nesting colonies of blue-morph geese were not discovered until 1929 after a direct and intentional search by Dewey Soper that lasted 6 years and covered 30,300 miles. Analysis of the Snow Goose mitochondrial DNA (mtDNA) genome across its range resolves two distinct clades, one concordant with geographic range, the other indicating no consistent geographic structuring; this suggests that two ancestral populations occupied spatially separated refugia during the Pleistocene. Evidence suggests the white- and blue-morph Snow Geese remained relatively allopatric in distribution until roughly 70 years ago, and since then, sympatry and gene flow have resulted in the current pattern of color frequencies. Until 1983, the 2 color morphs were considered separate species ( American Ornithologists' Union 1983 ). Structural size in the Snow Goose has been shown to be highly variable and dependent on growth conditions during the prefledging period. This phenomenon may be widespread among waterfowl and thus important in our views of the relationship between phenotype and genotype. If final adult body size of birds is heavily dependent on environmental conditions during early growth, as shown by Cooch et al. ( Cooch et al. 1991a ) and Lepage et al. ( Lepage et al. 1998a ), this has implications for the validity of body-size measurements and their reliability in the designation of infraspecific taxa.
The Snow Goose is a voracious forager, feeding for sustained periods of time in concentrated areas; females forage up to 50% of the day during spring migration and up to 75% on breeding grounds before nesting. On the wintering grounds, where the species is highly gregarious, it often feeds in very large flocks in restricted areas. Its diet consists entirely of plant material, and its primary foraging strategy involves grubbing for underground rhizomes, tubers, and roots, but it also grazes on tender new shoots of aquatic and agricultural plants, and during migration and on the wintering grounds actively scavenges agricultural fields for waste grain. As this species has taken advantage of previously unavailable sources of food on its wintering grounds and along migration routes, the population has increased rapidly—in some areas, as much as 9%/yr ( Gauthier and Brault 1998 ). Increasing populations, coupled with its primary foraging strategy of grubbing, is causing serious damage to its breeding and, in some areas, wintering habitats. Current estimates place its population at between 5 and 6 million, a number that may be environmentally unsustainable.
This species is socially monogamous, with life-long pair bonds formed usually away from the breeding grounds during the second year of life. During the breeding season, the species is aggressively territorial, with pairs defending an area of variable size around their nest site; during nonbreeding season, it is highly gregarious and flocking. The family unit is strong, with goslings remaining with the family until mate selection during their second or third year. Mating is assortative; birds in dimorphic populations actively choose mates according to the color of the family in which they were raised. What mixing occurs among nesting colonies occurs as a result of pair formation by birds away from their breeding colonies. As females are strongly philopatric, returning to the same breeding colony each year, there is little movement of females among colonies, but there may be considerable movement of males.
The Snow Goose is one of the most studied avian species in North America. Excellent intensive, long-term studies of breeding colonies, such as that at La Pérouse Bay, Manitoba ( Cooke et al. 1995 ), have given us a unique synthesis of the overall biology of this species. Other studies on the breeding grounds have contributed to our understanding of its food habits ( Gauthier and Tardif 1991 , Gauthier 1993b , Gadallah and Jefferies 1995 ), nesting requirements ( Alisauskas and Boyd 1994 ; Hughes et al. 1994a ; Ganter and Cooke Ganter and Cooke 1996 , Ganter and Cooke 1998 ), breeding biology (Davies and Cooke Davies and Cooke 1983b , Davies and Cooke 1983a ; Lank et al. Lank et al. 1989a , Lank et al. 1989b ; Schubert and Cooke 1993 ), brood-rearing behavior ( Mulder et al. 1995 ), and reproductive success ( Hamann et al. 1986 , Hamann and Cooke 1987 , Rockwell et al. 1993 , Viallefont et al. 1995 , Lepage et al. 2000 ). Studies at staging areas, migratory-stopover sites, and on the wintering grounds have clarified timing and routes of migration ( Alisauskas and Ankney 1992a ; Francis and Cooke 1992a ; Maisonneuve and Bédard Maisonneuve and Bédard 1992 , Maisonneuve and Bédard 1993 ; Baranyuk and Takekawa 1998 ) and food habits (Gauthier et al. Gauthier et al. 1984b , Gauthier et al. 1984a , Gauthier et al. 1992b ; Davis et al. 1989 ; Bélanger and Bédard Bélanger and Bédard 1994 , Bélanger and Bédard 1995 ; Ballard and Tacha 1995 ; Boyd 1995 ; Robertson et al. 1997a ). The genetics underlying color dimorphism in this species has been elucidated ( Cooke and Cooch 1968 , Rattray and Cooke 1984 ), as has spatial and temporal variation in color ( Avise et al. 1992 , Quinn 1992 ). Because of the Snow Goose's flocking behavior on the wintering grounds and during migration, we still do not know how, or exactly when, mate selection occurs. We lack vital information about the timing and location of molt in some populations and the molting process in general, and, although a highly vocal species, we know little about how vocalization develops and the context of various vocalizations.