It is an unusual thing to hear about a species you've never encountered and a truly exciting thing to actually see it with your own eyes. Recently, I spent some time in the San Francisco Bay area and bumped into a fellow birder who told me about a subspecies of Canada goose she had seen at a nearby golf course. She also mentioned there was a small flock of white-fronted geese (which I'd never seen) in the same grassy area and that they had been there just an hour before. I thanked her profusely and hurried to the area. I immediately found what she referred to as a smaller variety of Canada goose. This bird is called a cackling goose and is now considered a separate species (Branta hutchinsii) which includes four sub species. The common and well-known Canada geese (Branta candensis) includes eleven sub-species.
The primary physical differences between these two species is the size and the bill. Cackling geese are much smaller weighing 3 to 4 pounds with the largest subspecies weighing approximately 7 lbs. Canada geese, by comparison, range from 5 to 15 lbs. Aside from the size difference the bill size is pretty obvious. Cackling geese have smaller, stubbier bills (see below) making them stand out in a flock of Canada geese. Their voices are higher-pitched, which would be difficult to detect if you're ever heard a large flock of geese calling (honking)!
Note the size of difference (left photo above)
Note the bill difference (center photo above)
Can you find the cackling goose among the Canada geese in the last photo on the right? what about the greater white-fronted geese?