The newly recognized Cackling Goose is a smaller version of the Canada Goose. Formerly considered the smallest subspecies of one variable species, recent work on genetic differences found the four smallest forms to be very different. These four races are now recognized as a full species: the Cackling Goose. It breeds farther northward and westward than does the Canada Goose.
The Cackling Goose was long considered just a small race of the Canada Goose. The smallest four of the eleven recognized races were recently determined to be distinct enough to be their own species. Cackling Goose includes the races known as Taverner's, Richardson's, Aleutian, and Cackling geese. Confusingly, the "Lesser Canada Goose" is still a race of the Canada Goose.
Information from the All About Birds website, www.allaboutbirds.org, © Cornell Lab of Ornithology.