Lesser White-fronted Goose Anser erythropus
Wild Lesser White-fronted Geese have seriously declined in Europe and in an attempt to reverse this decline the Swedish Lesser Whitefronted Goose Project has been involved in a translocation programme. Between 1981 and 1999 a total of 344 birds were released using birds from captive stock. Only a fraction of those birds actually bred in the wild and it is not known to what extent they interbred with the existing wild stock. The releases stopped in 1999 when it was discovered that a small proportion (perhaps 5-10%) of the released birds were not genetically pure, having introgressed with (Greater) White-fronted Goose. Instead, since 2005 the Swedish project has used wild birds caught as goslings from the breeding area north of the Ural Mountains to build up a completely new captive breeding programme. Some of these Russian-origin birds have been released since 2010 with a view to reinforcing the wild Swedish population. Many thanks to Niklas Liljebäck of the Swedish Lesser Whitefronted Goose Project for clarifying and correcting the information given here.
Wild vagrants have appeared in the UK in the past and a tiny number of recent records (e.g. a bird with White-fronted Geese in Essex in the winter of 2016/17) are believed likely to be wild birds from Sweden or beyond. However the species is popular in captivity and escaped birds are much more frequently encountered in the wild, usually among feral geese (especially Greylags).
Whether the levels of genetic introgression found in the Swedish birds released prior to 1999 is representative of the levels of introgression in UK captive birds is not currently clear. It seems that there is some degree of variation among wild birds concerning features like the extent of the white forehead blaze, but my own experience of captive and escaped birds suggests that a reduced white forehead blaze is more frequent than I would expect it to be in wild birds. Whether that is because the birds have some (Greater) White-fronted Goose genes or not remains to be firmly established.
The first set of photos below show a bird that was considered likely to be wild among Taiga Bean Geese. The remainder are either birds in captivity or feral - some of these show anomalous head patterns to a lesser or greater extent and are therefore may not pure.