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.
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 upper presumed hybrid is from Oklahoma, the smallest-billed of the three (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, but Ross’s show a high rounded crown so that 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.
Hello,
I enjoy your web site.
I hope there will be a way to display a full discussion with comments and then print it out the text in full-page format.
Thank you.
16 December 2011
I came across a couple of Ross’s Geese mixed in with a large flock of greater snow geese at Chincoteague NWR on November 3, 2014. In going over some pictures, I think there is a hybrid snow X ross’s goose mixed in. It was clearly smaller than the snow geese, about the size of the larger of the two ross’s geese, with a bill that was in between in size. The head was rounder than the snow geese and the grin patch was there but smaller.
What do you think?
https://www.flickr.com/photos/73831614@N00/15705371906/in/photostream/
I’d appreciate any thoughts about this apparent immature hybrid Blue Morph: https://www.flickr.com/photos/bryanto/ . The bill appears intermediate, too long for Ross’s with some black lips, but not stout or long enough for Snow. The plumage is a close match for a Blue Ross’s, white wing coverts and belly, blackish back,belly and neck. Specifically wondering if both parents would have to be Blue morph to produce a hybrid this dark, or what insight this bird may have regarding the Blue genes.
Sorry, link to the photo of this goose here: https://flic.kr/p/roRMUF
Hi Bryant, I agree this looks like a hybrid. I don’t know the genetics, but I don’t think this is a simple dominant-recessive thing. Many decades ago the dark morph (Blue Goose) was almost entirely restricted to a population nesting in central Canada and wintering on the Gulf Coast. In the last few decades the population of Snow Goose has expanded, with different populations mixing, and the genes for dark morph seem to have spread. Dark birds are much more numerous continent-wide, and dark Ross’s were first recorded in the 1970s and seem to be increasing. My impression is that there are very few dark birds that show the features of pure Ross’s, and that dark hybrids outnumber them, but we can’t assume that the appearance of size and shape always matches the genes of the bird. It has been suggested that every “dark Ross’s Goose” is, by default, a hybrid, since the dark morph genes originated with Snow Goose.
I’ve noticed these changes in my birding life time. Every year Snow Geese migrate through Utah, this is a very specific sub-population that winter at the Salton Sea in California, and summer near the Queen Elizabeth Islands in the Canadian Arctic Archipelago, or at least that’s what the band recoveries indicate. Originally, before my time, they were all white Lesser Snow Geese, no Blues or Ross’s. However as Snow/Ross’s Geese adapted to agriculture and their populations began to dramatically increase, Blue Morphs and Ross’s began showing up in our sub-flyway. By the time I started birding in the mid 90’s, it wasn’t unusual to see a few Ross’s mixed in with the Snows, probably about a 500 to 1 ratio, and even fewer still Blue morph Snows, about 2000 to 1. In recent years however we have seen dramatic changes, Blue morphs seem to become more common every year, for instance last week I was looking at a flock of 2000ish Snow Geese, with at least 6 Blue morphs, all separate from another, as in not a single family unit. That’s a ratio of 1:333, which is a dramatic increase. Ross’s have increased even more dramatically, in many flocks Ross’s out number snows by 3 to 1,especially toward the end of their migration,implying Ross’s delay a bit. Never the less I have seen flocks with over 2000 Ross’s, so that our sub-flyway now has an overall ratio of about 1:20 Ross’s, and I have seen 2 apparent adult Blue Morph Ross’s, 1 last year in spring 2014: https://flic.kr/p/kKvsdg , another in the spring of 2010: https://flic.kr/p/7L4QeF . So 3 things have happened. All Snow/Ross’s geese populations have dramatically increased, what we have seen here is nothing compared to whats happened in the mid-west. I went to Nebraska in 2000, and saw MILLONS of Snow/Ross’s Geese, murmurations of them(about equally split between Blue/white morphs, but I wasn’t paying attention to how many Ross’s). Also at the same time Ross’s have dramatically expanded both their populations and range,originally mainly being found in the Pacific Flyway,they now are found across the continent, although still more abundant in the west. The 3rd thing that has happened, as all the populations have expanded, is Blue Morphs have become increasingly prevalent in all populations. An easy explanation for this is that there has been a lot of genetic swapping between the mid-west and pacific populations on the nesting grounds, but I speculate another possible origin of the “Blue Gene”. Many taxonomist contend that the genus Chen is really just a sub-genus within Anser, and whether you buy that or not, one has to say the 2 groups are each others closest relatives, and that Chen probably evolved from the Anser. If Chen evolved from Anser, then the Blue morph would be the ancestral form, and therefore its possible it could be present in all Snow and Ross’s as an ancestral gene.The fact the Emperor Goose only exists in the Blue morph supports the Anser origin of Chen. It could also have something to do with the amount of snow cover,as obviously the white morph originated as a means of camouflage during snow cover,especially probably on the nesting grounds. As the climate has warmed, and their populations expanded at the same time, perhaps there is some internal mechanism to select for the Blue gene in response to climate change,or more specifically lack of snow on the ground at the time of nesting. But I admits that’s all just speculation on my part. Anyway, its fascinating to see change in populations during our life time,and possible evolution in action.
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