Everything in nature has multiple connections, so in studying bird ID, we don’t just look at the physical field marks of the bird in isolation. We have to think about habitat, behavior, total distribution, and many other factors.
The Snow Goose provides a perfect example. The challenge of identifying this species has changed in intriguing ways during recent decades. It’s not that the bird looks different today; the change is driven by a huge population increase and range expansion of white geese in general.
Snow Geese occur in two color morphs: white (with black wingtips) and “blue,” with a mostly blue-gray body and white head. The two look so different that they were classified as separate species until 1973. Identification of the “Blue Goose” morph is seldom a problem, but it can be distinctly challenging to separate the white Snow Goose from a very similar but smaller bird, Ross’s Goose.
At one time, Ross’s Goose was scarce and localized, nesting in a limited area in north-central Canada, wintering mainly in central valleys of California. Prior to the 1950s, its total population was estimated at only about 6,000. Today it may number as many as two million. Its breeding range has greatly expanded, and it may be found wintering over much of temperate North America.
So in many areas where Snow Geese could once be identified at a glance, now we have to take a second look to rule out Ross’s Goose. Fortunately, there are distinct differences between typical Snow and Ross’s Geese. On the Atlantic Coast, in the wintering range of the Greater subspecies of Snow Goose, the few Ross’s Geese that occur look tiny by comparison. Farther west, the Lesser Snow Goose averages about 20 percent larger than Ross’s overall, but size alone isn’t enough for ID. There it’s important to study details of the bill shape and pattern, as described in the photo captions with this column. A prominent black “grin patch” marks the Snow Goose’s large pink bill where the mandibles come together. Ross’s Goose shows barely a hint of this; its bill is small and stubby and has a two-toned look, mostly reddish pink with a contrasting blue-gray base.
But there’s a new complication. Snow and Ross’s Geese have both gone through a tremendous population increase and range expansion, and their breeding ranges now overlap broadly. The two species often interbreed. Some studies have suggested that hybrids make up almost five percent of the total population, although others indicate that the proportion is below two percent. Furthermore, Snow X Ross’s hybrids have backcrossed with the parental species, so it’s now possible to see a full range of intermediates between “pure” Snow and Ross’s Geese. If we find one of these geese in an area where it would be rare, we now have to study every detail to be able to identify it with certainty.
What to look for
Size and shape. Larger than a Mallard, with fairly long neck.
Bill shape and pattern. Long triangular bill with black edges of mandibles creating a “grin patch.”
White morph. Adult: white plumage (often with orange staining on head) with black primaries. Juvenile: heavily marked with dingy gray; legs and bill blackish at first.
Blue morph. Adult: body dark blue-gray with variable amount of white on belly. White head, often stained orange. Juvenile: dark gray-brown, with legs and bill blackish.
The best-known field mark for Ross’s Goose is the fact that it lacks the Snow Goose’s black “grin patch.” However, everything about the bill shape and color is distinctive. Ross’s Goose has a short, stubby bill, often appearing “punched in” to the face. The bill is mostly pink, with a noticeable blue-gray patch at the base; in older birds, this blue-gray area looks quite bumpy or warty at close range. The border between the base of the bill and the feathering of the face forms a fairly straight vertical line. In addition, Ross’s Goose is a distinctly smaller bird and has a more rounded head and shorter neck than Snow Goose, contributing to a more gentle or “cute” facial expression.
The “Blue Goose” morph of Snow Goose, long considered a separate species, is distinctive as an adult, with white head and dark body plumage. The only similar species is Emperor Goose, an Alaskan specialty with a prominent black chin. However, there’s also a very rare blue morph of Ross’s Goose. Field marks of size, shape, bill shape, and bill color are useful for picking out this form. In addition, their body plumage tends to be blacker than that of the blue Snow Goose. They usually have white bellies (but so do many blue Snows). Most birds that look like blue Ross’s at first turn out to be Snow X Ross’s Goose hybrids, so this form must be identified with caution.
Young Snow Geese of the white morph show much dingy gray at first, especially on the upperparts, and their legs and bills are blackish early in fall. Their appearance changes gradually throughout the late fall, winter, and spring, as the gray fades and as new white feathers are molted in. On many, the bill begins to turn pink by November, but the December bird in this photo clearly still has a very dark bill. Domestic geese can show any number of variations, and some have a color pattern similar to this; so a purported Snow Goose seen alone, away from any flock, should be checked carefully for details of bill shape, overall shape, and contrasting black primaries in the wings.
First-winter immatures of Ross’s and Snow Geese differ in size, shape, and bill shape, just as adults do. This young Ross’s Goose shows the typical short-necked, round-headed, short-billed structure of the species. In addition, the overall plumage of the young Ross’s is paler to begin with, with a much lighter and less extensive wash of gray on the upperparts. This pale gray is replaced by white feathers through a gradual molt during the first season. Studies of migrating geese in the Canadian prairies have shown that the bills of young Ross’s Geese are already turning pink in October, when young Snows still have dark bills, so the bill color shown in this December photo isn’t unexpected.
Blue Goose mysteries
It’s not surprising that the “Blue Goose” was long considered a different species. This dark-bodied, white-headed morph is not randomly distributed through the Snow Goose population; it migrates through the center of the continent, and is generally uncommon east or west of that corridor. At one time, when its population was lower, it was even more tightly restricted in range.
In fact, as late as the 1920s, its nesting grounds were unknown to science. An intrepid Canadian scientist, J. Dewey Soper, spent several seasons searching remote areas of the Arctic for the birds without success. Finally, following tips from the Inuit, he visited the Bowman Bay area on the southwest coast of Baffin Island. Blue Geese were abundant there, and Soper found the first nest on June 20, 1929. A 3,000-square-mile tract of Baffin Island is now designated as the Dewey Soper Migratory Bird Sanctuary, and it’s one of the most important nesting areas for geese in Canada.
With the increase and spread of Snow Geese in general, Blue Geese also have become more widespread. But it was a shock to birders when biologists Robert and Ilse McLandress documented, in 1979, a blue morph in Ross’s Goose also. This “Blue Ross” is still exceptionally rare. Did the gene for the blue morph enter the Ross’s Goose population through interbreeding and backcrossing with Snow Geese? We still don’t know for sure.