The more you learn, the more there is to learn. This is true about every part of my life, but one of the clearest examples is the cackling goose (Branta hutchinsii). Cackling geese are very similar to the ubiquitous Canada goose (Branta canadensis) seen on ponds and lawns all over North America, so similar that I didn’t realize we had mixed flocks of cackling and Canada geese in Colorado every winter until recently.

I can’t remember a time when I didn’t know what a Canada goose looked like and I can’t remember when I learned to refer to it as Canada and not Canadian goose. (Canada is its name, not an adjective.) But this supposedly familiar bird turns out to have a huge range of distingiushable variations. Before 2004, they were all treated as subspecies. Then scientists decided to split out some of the smaller subspecies that breed on the tundra and name them cackling goose.

Now there are seven subspecies of Canada goose and four of cackling, and they are all only slightly different from one another. The Canada goose has an oval head and long bill compared to the cackling goose’s round head and stubby bill. The subspecies of both types are a series of gradations from large bill, oval head, long neck, and large body to tiny bill, round head, short neck, and small body. Actually identifying the specific subspecies of either a cackling or Canada goose is tricky, and the fact that the largest cacklers are nearly identical to the smallest Canadas doesn’t help any.

I saw my first cackling geese three winters ago. A flock flew overhead and I could tell at once they weren’t Canada geese, though the markings appeared identical. Their necks were noticeably shorter and their calls were higher pitched, sounding more like an excited cackle than a sonorous honk. It was exciting to get them for my life list, but even then I didn’t realize how ubiquitous these geese are during the winter in Colorado.

Now that I live on a lake, I see Canada and cackling geese daily. I no longer take these geese for granted and have to look at each individual carefully, noting as best I can length of neck, bill shape, and body size, hoping it will be clear to me if what I see is Canada or cackling. (I haven’t had the gumption to try to sort out all the subspecies!) I’m always grateful to see a really large (Canada) or small (cackling) goose because then I am fairly confident of what I am seeing.

Knowing about cackling geese has made my goose watching more complicated but also more interesting. It reminds me that there is always more to learn, and that only by observing closely can we hope to really see and appreciate the subtle variation nature is capable of.

Have you ever been surprised to discover the variation within or between species?