Swan Goose hybrids Anser cygnoides x Anser/Branta sp.
I think it's fairly safe to assume that the first bird below is a hybrid between a Barnacle Goose Branta leucopsis and one of the grey geese (Anser sp.). The all-dark bill would be unexpected if Greylag Goose was the second parent but that together with the long bill and straight bill/head profile, seems to point instead to Swan Goose, so this bird is considered most likely to be a Swan Goose x Barnacle Goose hybrid. The bird was alone so judging size accurately was impossible, however the impression was of a fairly small (and short-necked) goose. Other Swan Goose x Barnacle Goose hybrids have been notes as being larger, so the identity of this bird is not entirely certain.
probable Swan Goose x Barnacle Goose, Welney (Norfolk, UK), 28th March 2009
The identifications of the following birds are more uncertain. The first one was with about 100 feral Greylag Geese (one domestic). The dark crown and back of the neck is typical of Swan Goose and a white surround to the bill can be present with hybrids of domestic Swan Geese. It lacked the bump at the base of the bill that is often found on domestic Swan Geese, but had the rather straight head/bill profile that's typical of wild-type birds of that species. It seems likely that Swan Goose was one of the parents, and although some of the features of domestic birds are absent, the likelihood must be that it was a domestic bird.
The bird was distinctly smaller than the accompanying Greylags and very short-necked, which suggests that if Swan Goose was one parent then a small species should be the other - but none of the very small options seem to provide a satisfactory solution. The body feathering were paler than on the Greylags, with a slightly silvery sheen and this, together with the bill colour may well indicate Bar-headed Goose Anser indicus - the neck pattern may also be a feature that came from this parent as Bar-headed Goose hybrids often show this (juvenile Bar-headed Geese have a similar pattern).
So disregarding size and structure Swan Goose x Bar-headed Goose seems a good bet, but in view of the small size and short neck there is some doubt over that ID.
possible Swan Goose x Bar-headed Goose hybrid, Swanton Morley (Norfolk, UK), 25th February 2007
I have struggled with the ID of the following bird and thanks to Joern and others for their input. It seems to have Swan Goose in it, and the broad white tertial fringes suggest Snow Goose or Ross's Goose. Possibly it is domestic Swan Goose x Snow Goose, or maybe it is the progeny of a domestic Swan Goose and this possible Ross's Goose x Lesser White-fronted Goose hybrid?
Domestic geese are descendents of either Swan Goose or Greylag Goose, or both. Many show clear evidence of both species and can therefore be accurately described as Swan Goose x Greylag Goose hybrids, however many others are impossible to identify with certainty. Hybrids are fertile and the influence of one or other species may be from several generations back in their ancestry. Moreover it is not entirely clear whether features normally associated with one species may sometimes appear in domestic variants of the other species not through hybridisation but as a result of genes inherited from a common ancestor being activated in the domestic form where they are dormant in the natural form. For this reason I have not devoted space to this hybrid here, instead including photos of domestic hybrids on my domestic goose page.