When I saw a hybrid goose x ? a few years ago at Ridgefield NWR, most of the local birders suggest Canada x Greater White-fronted Goose, whereas those on the East Coast and Europe suggested a Canada x Greylag goose or domestic goose mix. I am of the opinion that proximity makes the Greater White-fronted Goose a far more likely candidate than the Greylag. Think of where the geese are when they are migrating, pairing off and mating. In Europe there are far more Greylags around. The geese here are far more likely to run into a Greater White-fronted Goose than a Greylag.