Kentucky Ornithological Society

Distinguishing Ross's and Snow Geese and Their Hybrids

David Roemer

Ross's Goose (Chen rossii) is a regularly occurring and often overlooked migrant and winter resident in Kentucky in small numbers and is usually found in the company of Snow Geese (Chen caerulescens).  Identification of these species is relatively straightforward based primarily on structural differences, particularly those of head and bill shape.  Hybrids of these species are sometimes encountered and are intermediate in structure between the two.

Although Ross's Goose is usually smaller than Snow Goose, size in itself is not diagnostic.  It is a good clue, however, and noticeably small birds in a flock should be looked at closely when searching for Ross's.  Primary structural differences of Ross's Goose as compared to Snow Goose include:

In the Identification Guide to North American Birds Part 2, Pyle states the range in size of the gap along tomia in Lesser Snow Goose as 7-12 mm wide and in hybrids as 4-9 mm wide.  The gap between mandibles in Ross's Goose is narrower forming no or a thinner dusky stripe along tomia.  Many Ross's Geese show a slight gap between the mandibles.         

The plumage of Ross's Goose is typically very white and rarely attains the rusty staining which Snow Goose often does, particularly about the head and neck.

Hybrids are best identified by intermediate head and bill characters such as bill size and shape, line of feathering at bill base, degree of bevelling or gap along tomia, and varying amounts of grayish at bill base which Snow Goose lacks.

Images depicting comparisons of and differences between the species and hybrids are shown below and all were obtained in Kentucky.  Flight images were obtained on 2 February 2007 in Fulton County, and the remainder were obtained in Warren County between 2003 and 2009.

Image A shows a first cycle light morph Snow Goose retaining some dusky juvenal plumage. 
Note the elongated head and bill with wide bevelled gap between mandibles.   

Image B shows a Ross's Goose.  Note the rounded and less elongated head and more vertical feathering at the base of the bill. 
The bill is short and stubby with almost no gap between the mandibles.  Note also the grayish base to the upper mandible.

Image C shows a Ross' x Snow Goose hybrid.  Note the intermediate head and bill characters including gap
between mandibles and grayish base to the upper mandible.   

Image D is a digiscoped shot showing 3 Ross's Geese followed by a Snow Goose along with Canada Geese.  Differences in
size and structure between the Ross's Geese and Snow are very apparent in this image.  Note the dark line before the eye
in the rear pair of Ross's which is retained juvenal plumage.  Juvenile Ross's Geese typically show far less duskiness to
their plumage than juvenile Snows, and a dark line before the eye is the most consistent plumage character suggesting
immaturity in Ross's Goose and can be seen at a surprising distance even in flying birds.   

Image E is of a dark morph Snow Goose showing head and bill detail.  Note the width of the bevelled gap between the mandibles. 

Image F is of a first cycle light morph Snow Goose showing head and bill detail.   

Image G is of a Ross's Goose showing head and bill detail.  Note the rounded and less elongated head as compared to
Snow Goose along with the more vertical feathering at the bill base.  Also the stubbier bill with dark grayish base to
the upper mandible and narrow gap between the mandibles which almost totally lack bevelling. 

Image H is of a Ross' x Snow Goose hybrid showing intermediate head and bill characters.  Note the reduced bevelled gap
between the mandibles as compared to Snow Goose and the darkish area at the base of the upper mandible from Ross's Goose. 
The bill is longer than that of Ross's and the feathering at the base of the bill is intermediate between parent species.   

Image I is of a flock of Snow Geese as they might appear overhead.  A closer look reveals 3 Ross's Geese and a hybrid in this flock.   

Image J is a crop of the 5 birds center-right in the image above, or in the front of the flock.  This image approximates the
look that one might have through the binocular or scope.  The group is comprised of 3 Snow Geese and 2 Ross's Geese (birds
banked without shadows on their bellies).  Difference in size between the species is not apparent here but structural differences
are very much so.  Note the shorter necks and more rounded, less elongated heads of the Ross's.  The more vertical line of feathering
at the base of the smaller and stubbier bills of the Ross's gives the appearance of having the face cut off at the front.  From a distance
in flight, the bill of Ross's often virtually disappears and the vertical line at the front of the face is very apparent.

Image K is also a crop of the same image showing birds in the lower left or at the rear of the flock.  This group is comprised
of 4 Snow Geese, a Ross's (bottom center) and a hybrid (second bird from top left).  Note the short neck, rounded head with
vertical line of feathering at the bill base and short stubby bill of the Ross's.  This bird is also smaller than the others in this
group.  The hybrid shows a shorter, rounder body and shorter neck as well as a less elongated head than the Snows, but the
feathering at the bill base is convex lacking the vertically cut off look of Ross's and is intermediate between the species.  If
one looks closely, there appears to be a relatively wide and dark gap between the mandibles which would not be as apparent at this
distance in Ross's. 

Image L is closer crop of the hybrid and trailing Snow Goose showing differences in structure.  The body and neck of the hybrid
suggest Ross's Goose but the head and bill are clearly intermediate between the species.  The dark gap between the mandibles can
be seen better in this crop. 

Image M shows a Ross's and two Snow Geese.  There is a noticeable size difference between this Ross's and the Snows and the
structural differences are very apparent.  As pertaining to the Ross's Goose, note the shorter, rounder body, shorter neck, less
elongated head with vertical feathering at the bill base, and short stubby bill with no apparent dark gap between the mandibles.    

Image N is a closer crop for comparison of the structural differences between Snow and Ross's Geese which are apparent in flight.

Of course, it is not practical to believe that one would have time to scrutinize or identify every bird in every flock flying overhead, but with practice and good views, some Ross's Geese and hybrids can be identified in flight based on structural characters.         

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