Ross's Goose Anser rossii (or Chen rossii)
The origin of most European records of Ross's Geese is uncertain. They are certainly a potential vagrant from North America but the situation is complicated by escapes from captivity, some of which are now breeding ferally in the UK.
The winter of 2003/2004 saw at least three individuals amongst the flocks of Pink-footed Geese wintering in Norfolk. At least one of these was an escaped bird since it bore a plastic ring, however this bird spent most of its time with feral Greylag Geese and was only occasionally seen with the Pink-foot flocks. The other two were probably wild vagrants, but it is impossible to be certain. One of them had also been present during the previous two winters; when it first arrived in autumn 2001 it was aged as a first-winter, strengthening the case for it being a wild vagrant. Since then one or two Ross's Geese have been present most winters and in 2007/2008 up to five potentially wild birds were reported, as well as one or two presumed escaped birds still knocking around.
The first few photos below are birds in wild goose flocks which may well be wild vagrants; then follow birds preferring the company of feral Barnacle Geese and these are most likely to be feral. At the bottom of the page are some birds still in captivity.
The following individual shows two dark secondaries in one wing and three in the other. I'm not sure if these are remnants of immature plumage making this bird is a second-winter, or if it's just variation within adult plumage. It seems that it is at least not uncommon in otherwise adult-like birds, but whether it ever occurs on older adults or not I am still uncertain. If anyone can throw any light on this I'd be interested to hear from you.