Taming the Tigers

So here is your guide to taming these tigers. I’ll begin by presenting the three eastern species merely by their distribution maps because, frankly, to most people (but soon not you among them) the three all look alike. (Maps courtesy of GBIF.^[1])

Differentiating Canadian and Eastern Tiger Swallowtails

Across vast regions of their respective ranges, Canadian and Eastern tiger swallowtails do not overlap — and therefore can be identified with ease. Although we don’t like to define species strictly by their ranges, it’s safe to say that if you’re in Alabama it’s an Eastern Tiger and if you’re in Alberta it’s a Canadian Tiger.

Identification becomes more complicated where the two ranges overlap or intersect: a zone generally stretching from New England through southern Ontario and into the Upper Midwest. Moreover, in the overlap zone we get hybrids (that we do not call Appalachian Tiger Swallowtails).

So, first, here below are essential distinguishing marks for Canadian and Eastern tiger swallowtails, including some you won’t find in field guides. The best marks are ventral (viewed from below). Dorsal-only identification can be fraught (more on this later).

Yeah, But …

So maybe this isn’t so complicated after all? Just get yourself a decent look at the tiger’s underparts. Well, uh, this is entomology, after all, which is nothing if not replete with wonderful complications. Here’s the problem: tiger swallowtails vary in appearance for various reasons, including where they live and perhaps what they ate as caterpillars. That Eastern Tiger Swallowtail above, for example, shows clear and extreme examples of most diagnostic marks. In truth, in the field, these two species can look more alike than what I’ve shown.

Early-spring Eastern Tigers Swallowtails, for example, look more like Canadian Tiger Swallowtails. And up here in northern New England, where Canadian Tiger Swallowtails dominate, we see introgression of Eastern Tiger Swallowtail genes. Basically the two species show mixed genetics owing to hybridization and other gene flow in our zone of overlap. And those in-betweeners seem to be becoming more common.

Finally, those ventral field marks do not express themselves dorsally, which is how lots of people see and photograph these butterflies. As a result, on iNaturalist (where I hang out and you should too) tigers are sometimes identified incorrectly based solely on dorsal photos. (So those of us vetting iNat butterfly records need to be more careful, and in some cases probably shouldn’t rely on dorsal views alone.)

There’s lots more to say and learn and read on these tigers, particularly this hybridization business (some of which you’ll find in the readings list below). For now, however, here are more image pairs: Canadians on the left and Easterns on the right. That final row of images, each from Vermont, shows tigers with apparent mixed lineages. It’s probably best to click and read captions rather than mouse-over, and then use your arrow keys to move forward or backward through images. (I’ll add dorsal identification advice and images at a later date.)