Great Kiskadee

Great Kiskadee

Tuesday, 27 March 2018

Ross's Goose...or ?

Ross's Goose has been considered for a long time as one of the rarest species of breeding goose in North America. The first evidence of this species breeding in Ontario was in 1975, when two of my former MNR and now retired colleagues (Paul Prevett and Fred Johnson) observed flightless young at Cape Henrietta Maria along Hudson Bay while they were working out of Moosonee. The population has continued to expand. During the second Ontario Breeding Bird Atlas, it was recorded with breeding evidence in 6 atlas squares.

While it is still a rarity, it has been seen much more regularly in the last couple of decades than before. One just has to look at the number of reports in Ontario on eBird, Ontbirds or other rare bird reporting sites to see just how much more frequently it occurs. Of course part of that is due to many more birders being out in the field and with higher quality optical equipment than several decades ago.

Ross's Goose nests in Snow Goose colonies, and as the latter species has expanded considerably in Ontario over the last few decades, it isn't all that surprising that Ross's Goose has expanded as well.
Nesting 'blue' phase Snow Goose, from scan of 1990 slide taken in Polar Bear Prov Park
Waterfowl are known to hybridize with regularity when the opportunity arises. Hybridization between Snow Goose and Ross's Goose may occur relatively frequently since they nest in the same colonies. With Ross's Goose looking like a smaller version of Snow Goose, it is possible that the two species think that as well, which facilitates hybridization. And that presents a problem for birders who want a clear cut species in order to put that check mark on their lists.

Recently two small white geese have been present at the Keith McLean Conservation Area, just outside of Rondeau Provincial Park. The high water levels and greater snow melt than usual allowed a fairly large but shallow pond to form at the south end of the CA, much of which was easily viewable from the adjacent road. There has been a typical mix of waterbirds present which vary from day to day, including Canada Geese (of course!), Northern Shoveler, Northern Pintail, American Wigeon, Gadwall, Mallard, Green-winged Teal, and others.

This next photo shows one of the birds observed by several folks. The photo was taken with a lens magnification of about 14X, so it is magnified a little higher than the average 8 or 10 power binocular view would provide. At first it looks good for a Ross's, being a small goose (that is a Ring-billed Gull out-of-focus in the foreground), with a round head, no obvious grin patch, a bluish area to the base of the bill and a fairly straight feather line at the base of the bill. Ross' Goose...check.
A few other shots, including the two of the birds.


Being chased by a probable Herring Gull


Next is a bit closer look, zoomed in. But with this closer view, questions start to arise. The bill looks a bit longer than a pure Ross's Goose, which normally has a short, stubby bill. And there is a hint of a grin patch, which Ross's should not have.
Zooming in even closer, the bill clearly does not look good for a pure Ross's Goose. If birders had viewed the birds with a 'scope and not just binoculars, they might have had the same questions come to mind.
After careful consideration, I changed my initial conclusion of Ross's Goose on eBird to a hybrid between Ross's and Snow Goose. I'm not sure how many others who checked off Ross's Goose will do the same, but the fact is that neither bird is a pure Ross's and to be accurate, should be recorded as a hybrid.

A few years ago I had taken photos of what I was initially convinced was a Ross's Goose out near St. Clair NWA and posted it as such along with a photo. I was contacted by a waterfowl biologist who had considerable experience with Ross's and Snow geese, among others, and he pointed out some of the less obvious characteristics that indicated the bird in question definitely had a mix of both species. Since that time, I have tried to be more careful when indicating on my lists which species I am reporting, as in many cases, the bird(s) show hybridization.

Does it really matter? It is apparent that this bird is 'mostly' Ross's....not 100% but still a beautiful bird. Nonetheless many checklists on eBird will likely remain as Ross's, and therefore muddying the true picture of Ross's vs hybrid.

With this in mind, it is likely that many Ross's Geese reported on eBird and other reporting sites are actually hybrids.






No comments:

Post a Comment