Ross's Goose x Emperor Goose Anser rossii x Anser canagicus (or Chen rossii x Chen canagica)
This hybrid appeared among Norfolk's Pink-footed Goose flocks in each of the five winters 2003-2007. Although regularly misidentified as a Blue Snow Goose by optimistic observers, it was in fact a hybrid.
The identity of this hybrid was the object of much debate but a confident consensus was reached among many competent observers that it was Ross's Goose x Pink-footed Goose and it was regularly documented as such. The assumption that Pink-footed Goose was one of its parents was presumably based on the fact that it accompanied that species - I can see no other reason to even consider Pink-footed Goose. The non-white parts of the plumage lacked the brown tones of Pink-footed Goose and the pattern of these feathers was different: the dark sub-terminal bar on the scapulars was more distinct and had more of a point at the centre. Surely if this bird had not been with Pink-footed Geese no-one would have even considered that species as a parent. The detail was much more akin to Emperor Goose than any of the grey geese.
A number of features suggest Snow or Ross's Goose involvement. The structure and especially the bill look good for these two (or Emperor Goose); the prominent white edges to the tertials and secondaries resemble those of blue phase Snow or Ross's Geese and of course the extensive white plumage is best explained by Ross's or Snow Goose involvement. If Pink-footed Goose was a parent then the large size would perhaps favour Snow Goose over Ross's Goose, but the largest Ross's Geese are as large (including a first-winter bird that first arrived among the wintering Pink-feet in autumn 2001). The pink bill shows a dusky area towards the base which suggests Ross's Goose, but this could have come from the other parent. The size and shape of the bill is closest to Ross's Goose, though it often appeared slightly longer. The base of the bill was relatively straight, not as curved as in Snow Goose.
If we assume this bird was of wild origin, then the consensus identification of Ross' Goose x Pink-footed Goose did seem the most likely. However, with nothing to suggest Pink-footed Goose over any other grey goose, this identification should always have been regarded as tentative at best. The evidence was never overwhelming and the confidence in this identification that existed in some parts of the birding community was, in my opinion, always misplaced.
The possibility that this bird was in fact Ross's Goose x Emperor Goose was raised several years ago by Joern Lehmhus, whose opinions on hybrid goose identification should always be listened to!
Relevant to this bird's identification are two very similar birds that were photographed in the Netherlands. These differed in having a small dark spot behind the eye and more orangey-coloured legs, but were otherwise extremely similar. The identification of those birds has been discussed in detail on a Dutch forum, waarneming.nl (in a mixture of Dutch and English).
A significant recent development on the discussion about the identity of all of these birds is an article dating back to 1979. Thanks to Joern for digging out The Journal of Heredity 70:395-400 (1979): Chromosome homology between the Ross's and the Emperor goose by R N Shoffner et al. This paper contains black-and-white photos of captive Ross's and Emperor Geese, a male and two female known hybrids between Ross's Goose and Emperor Goose as well as a further backcrossed hybrid. Two of the F1 hybrids in particular look extremely similar to both the Norfolk bird and the Dutch birds (one with the spot behind the eye like the Dutch ones and one more like the Norfolk bird). The resemblance is so close that it now seems hard to escape the conclusion that the Norfolk hybrid was indeed a Ross's Goose x Emperor Goose hybrid as Joern had suggested all along.
With Emperor Geese and their hybrids common in captivity the probability now has to be that this bird escaped from captivity. Having said that it is not inconceivable that a vagrant Emperor Goose could find its way to a Ross's Goose colony and produce hybrids, and there is some evidence to suggest hybrids are more prone to vagrancy than non-hybrids, so perhaps there is still a remote chance it was a wild bird!