Every once in a while the question “Canada goose or Canadian goose?” is used as yet another pedantic shibboleth, and I am pleased to find a birding page by Lisa Shea that addresses the issue with good sense and as scientific an attitude as any linguist could ask:
The vast majority of English speaking people call the goose that is large and has a black head—Branta canadensis—a Canadian Goose. However, its original name was a CANADA Goose.
Remember, the official name for any bird is its Latin name. So the “real” name for this creature is Branta canadensis. That’s because the bird probably has 200 different names in 200 different languages, based on its colors, its sounds, its habitat or many other reasons. Birds get named after people, after habits, after all sorts of things. The Latin name is the same around the world for that bird.
So it’s true that at one point in time the Branta canadensis was called a Canada Goose, because it was often seen flying towards Canada and living there. You could now just as easily call it a North American Goose since it is found all over North America and lives just about anywhere. It has adapted to live all across the US and into Mexico too.
So over the years, the name has changed to be Canadian Goose in English. Just like people in the 1600s used to call pumpkins “Pompions” and call vegetables “potherbs”, we have changed what we typically call the Branta canadensis to Canadian Goose.