There are two types of ‘feral’ – or naturalised – geese found widely across the UK. The Greylag Goose is the ancestor of the farmyard goose, and has a genuinely wild, mainly-wintering population, found in Scotland. The Canada Goose is almost exclusively derived from introduced stock. These introductions of the larger race canadensis from the east side of North America, began as long ago as the later 17th Century, during the reign of Charles II. So, the birds have been around much longer than ‘accepted’ introduced species, such as the Little Owl.
Why Canada? The bulk of the original (natural) breeding population is in the northern half of North America, principally Canada. With naturalised populations in the UK, New Zealand and South America, there is a little slice of Canada all round the world.