Identification of white geese

Recent discussion of the identification of a white goose in Ohio (beginning here) prompted me to put together some sketches of bill shapes of Ross's and Snow Geese, since bill shape is the most significant difference between those species. It was a very interesting exercise, as each species is surprisingly consistent in the relative size and proportions of head and bill, suggesting that the overall impression of bill size is a fairly reliable (though subjective) feature, and there are some other details that allow a more objective identification.

Tracings of head and bill shapes of white geese, adjusted so head sizes match, to emphasize differences in proportions. Original pencil sketches copyright David Sibley.

The middle column of presumed hybrids is traced from three individual birds. In the middle is one from Kentucky (photograph by David Roemer here) that seems to be a straightforward intermediate bird and almost certainly a hybrid. The top bird in the presumed hybrids is from Oklahoma (traced from a photograph by Victor Fazio here) and the bottom presumed hybrid is the contentious Ohio individual (traced from a photo by Matt Valencic here).

Besides absolute bill size, the features that seem most useful for distinguishing Ross's from Snow and from potential hybrids are:

faint or absent "grin patch" – Ross's usually show a small and inconspicuous dark line, Snow Geese an obvious black oval. This is related to the following…
lower mandible nearly straight on Ross's, strongly curved on Snow, and slightly curved on hybrids
border of feathering at base of bill relatively straight on Ross's, curved on Snow – this is somewhat variable in Ross's, and seems even more variable in hybrids (if these three are really hybrids), but Snow always has the border strongly curved, and Ross's straight or slightly curved.
As a measure of bill length, on Ross's the bill is always obviously shorter than the thickness at the top of the neck, on Snow the bill length is greater than neck thickness, and hybrids intermediate.
Round head – There is little difference in forehead slope or crown shape, the perception of a different head shape seems to come from the fact that the head of Ross's can be described as a circle, while on Snow Goose (and hybrids) the head is more oval.

I still maintain that the Ohio bird fits into the "hybrid" column better than the Ross's column. There must be backcrosses and maybe even pure Ross's Geese that blur the distinction between these categories as illustrated, and discovering that will require a more detailed study of variation in a large number of geese. Hopefully these sketches will help the discussion move forward.