which features are useful? (2015 & 2017)

by Vincent van der Spek & Fred Visscher


- Blue Ross's Geese are globally very rare


- Do they even exist?

- Unravelling the identity of a bird reported in the Netherlands


- Markers to ID hybrids


Unlike surrounding countries, Ross's Goose (Anser rossii) is regarded a species with vagrancy potential in the Netherlands. To get a bird accepted, the rules are strict: good photographic evidence is needed in order to proof a bird is unringed and fully winged.


In the UK for instance, all birds are considered escapes. Though I'm sceptical about individual sightings, both points of view seem defendable. They are not uncommon in captivity and the list of proven escapes is extensive. On the other hand, once rare, the global population has increased enormously in a short time: from 2000-3000 individuals in the early fifties to about 2 million in 2015 (Reeber 2015). It's a long distant migrant and for many Nearctic wildfowl species (including geese) vagrancy to Europe is proven by rings. So why treat Ross's Goose any different than other Nearctic species that are common in aviaries?


But a sighting in the autumn of 2015 particularly triggered me, since it was of the blue form. It was twitched by quite a few birders, but on pictures of the bird something looked off to me. I contacted wildfowl enthusiast Fred Visscher to share my thoughts, and together we analysed the bird. We came to the conclusion it was a hybrid Ross's x Snow Goose (f1+). Several American ornithologists we contacted supported this point of view. Consequently, the bird was not accepted by the Dutch rarity committee CDNA.


Very rare

Why did I even bother to take a second look at this bird in the first place? Well, blue Ross's Geese are extremely rare. Only 3 out of 38.825 birds in a survey carried out in the seventies were blue (McLandress & McLandress, 1979). With the increased Ross's Goose population the number of blue birds must have grown (and note that the percentage of blue Snow Geese (Anser caerulescens) has increased over time, but more about that later), but still: they are rare, really rare. That there currently are 100s of blue Ross's Geese seems more likely than 1000s.


What's a blue Ross's Goose anyway?

But do they even exist? Historically blue Ross's Goose is unknown. In Snow Goose, an allel for the blue form was found. Such an allel hasn't been found in Ross's Goose. The most valid explanation is hybridisation (and back crossing) with Lesser Snow Goose, which is known to occur. The “blue allel” in Snow Goose is partially dominant: that's why they increase. This increase is less strong than the models predicted, but that's because they are suspected to be a little racist: they seem to prefer a partner of their own colour.


Other explanations for blue Ross's Goose are that i) the allel hasn't been found yet, ii) that it is caused by a yet unknown gen mutation, iii) the survival rate of blue chicks is much lower.


I won't get into this too deep, but when using Oxham's razor, a hybrid origin (undoubtly with back crosses) seems most likely: it's the explanation that raises the least amount of questions.


So if a hybrid origin is the cause, it must be possible to find Snow Goose characters in presumed blue Ross's Geese (I avoid the almost philosophical question: 'after how many generations of back crossing with pure birds can you call it a Ross's Goose again?' Oxham is enough philosophy for the day). It's fairly easy to find birds reported in the US that obviously have Snow Geese genes that. Good birders over there are of course fully aware of the situation. E.g. check this post.


The contradiction: what should a blue Ross's Goose look like?

This is a bit of a contradiction after what I just wrote: if the most likely hypothesis ("all blue Ross's Geese have Snow Geese genes") is true, then there is no 'classic' plumage. But we had to start somewhere when we analysed the Dutch bird.


Some of the features we found are questionable. If so, this is mentioned.

Of course the experts on the matter are Americans. We used Madge & Burn (1988), Sibley (2014; as well as this fantastic web post), this weblog and, the only European in this list, Reeber (2015).


Dutch bird autumn-spring 2015/2016 and 2016 /2017

We've summed up what we think is either wrong, or at least raises questions.


NOTE: of course blue Snow Goose is variable, so why would that be any different in Ross's Goose? True, but especially in a vagrant context, in a rare bird with an even rarer plumage, a bird should be undisputed!


The features described below are based on the afore mentioned literature and knowledgeable bloggers, the added notes are based on 100s of Snow and dozens of Ross's Goose pictures and/or comments from American birders.

Probable Ross's x Snow Goose hybrid, Horstermeerpolder, Noord-Holland, The Netherlands, December 2015 (Ruwan Aluvihare)

  • Overall structure: the bird seems to be trifle too large, with a fairly long neck. The forehead doesn't look steep enough.


  • The bill seems to be too long and too thin. Right in front of the nail the culmen is a little concave. This should be straight in Ross's Goose. The grinning patch seems to be on the larger side for Ross's Goose. In the best shot, the lower mandible doesn't look straight, though it's hard to judge. The border of the feathering at the base of the bill should be nearly straight in Ross's and curved in Snow. In this bird the feathers are slightly curved, though perhaps still within the variation of Ross's. There's a little grey at the base of the bill, but Ross's Goose should show this more extensively: this is a described feature for hybrids (Reeber 2015). And while sex and age dependent (most striking in adult males), there are no warts on the culmen. The bill looks pretty much like the one from this hybrid (lower right), though the Dutch bird has an even more slender bill.


  • Reeber (2015) describes an interesting feature I haven't come across anywhere else: Snow has an oval shaped eye, while it's more rounded in Ross's. Checking pictures on the internet I see a little variation, but in general it seems right: the most pronounced oval eyes do not seem to occur in Ross's. The Dutch bird however, does have an oval shaped eye.
    Note: interesting feature to get into, though none of the consulted American birders mentioned itIn a Dutch zoo with seemingly pure Ross's Geese, I noticed at least two birds with more oval shaped eyes (July 2017).

Probable Ross's x Snow Goose hybrid, Beugen, Noord-Brabant, The Netherlands, 
October 2015 (Peter van de Braak)

  • Blue birds 'most frequently' (Reeber 2015) have black on the crown. The Dutch bird has a white crown, like blue Snow Goose.


  • There's white on the chest and neck. Shouldn't this be all black? This is a regular feature in Snow Goose.


  • In Ross's Goose the pattern of the elongated greater coverts is usually different compared to blue Snow Geese. These are usually white with a dark centre (around the shaft). In Snow Goose it's the other way around. Most of the feather is dark, with small white edges (Sibley 2014; Reeber 2015). This bird shows feathers typical for Snow Goose.
    Note: a lot of relatively unsuspicious birds indeed show this character, but we did find structurally (and otherwise) good looking birds with feathers comparable to the Dutch bird. None of the American experts mentioned this. How useful this feature can be, remains unclear.


  • The other wing coverts of typical birds are white or whitish. In the Dutch bird they are grey, like in Snow Goose.
    Note: see previous note. None of the American experts mentioned this. How useful this can be, remains unclear.

Probable Ross's x Snow Goose hybrid, Beugen, Noord-Brabant, The Netherlands, 
October 2015 (Peter van de Braak)

Expert opinions

Several knowledgeable American birders were asked for their opinion (I did not share our analyses beforehand in order not to push them in any directions): Kiel Drake (a Ross's Goose reseacher), Alvaro Jaramillo, Steve Mlodinow, Ray Alisauskas (Ross's Goose reseacher) and Joseph Morlan were kind enough to reply.


Their comments show how difficult this is: most of them wrote that others might think differently about this bird. But they didn't. They all thought that this bird was a hybrid. All five mention the bill structure (see my analyses above). Two of them mentioned the (little) amount of grey at the base of the bill and two of them mentioned the white parts on the neck and breast. None mentioned the other plumage features (white crown; coverts), the warts or the shape of the eye.  



Fred, 'the Americans' and I all see markers for Snow Goose genes in this bird. Based on our findings, the bird was rejected by the Dutch rarity committee. The shape, size and colouration of the bill are the most important features. The overall structure also seems a little off. How useful the shape of the eye and the plumage features are remains somewhat unclear, but they are not in favour of this bird. A hybrid seems to be very likely – if there's something like a pure blue Ross's Goose in the first place!

Ray Alisauskas, Arnoud van den Berg, Max Berlijn, Kiel Drake, Nils van Duivendijk, Alvaro Jaramillo, Steve Mlodinow, Joseph Morlan and Peter de Vries are kindly thanked for sharing there thoughts, or by getting in touch with contacts in the States. Our first thoughts on this bird were shared on



McLandress, M & L McLandress, 1979. Blue-phase Ross’ Geese and other blue-phaes geese in western North America. Auk.


Madge, S. & H. Burn, 1988. Wildfowl. An identification guide. Helm publishing.


Reeber, S. 2015. Wildfowl of Europe, Asia and North America. An identification guide. Helm Publishing.


Sibley, D.A. 2014. The Sibley guide to birds. Bloomsbury Publishing PLC.