Wild Side: Eastern tiger swallowtail

This has, by and large, been a pitiful season for butterflies. As I wrote in my July 12 column, I believe butterfly numbers in general are in steady decline on the Vineyard, and in most of the world that I’m familiar with. And the vicious drought that the Island has experienced this summer has wrought havoc on many types of herbivorous insects, depriving them of the fast-growing, tender, and nutritious vegetation that optimizes their larval growth and development.

Yet, as often happens, several species among our butterflies have somehow pulled off bumper crops. A conspicuous example would be the Eastern tiger swallowtail, Papilio glaucus, which has been present across the Vineyard in numbers I haven’t seen for years.

I won’t complain. This species, large and dramatically colored, ranks among my favorites. The largest tiger swallowtails may boast a wingspan approaching six inches, though 4½ is probably more like the norm. True to its common name, tiger swallowtails are yellow with bold black striping. The topside also shows extensive blue on the hind wings, and viewed from below, the hind wings are edged with a row of orange spots. Like swallowtails generally, P. glaucus features projections extending off the rear of the hind wings — the eponymous swallowtails, suggesting the forked tail of a barn swallow — though these often break or wear off. Still, this is not a species you’ll confuse with anything else.

In other portions of the East, ID is complicated by a couple of closely related species, so closely related that they were once all lumped together. The three taxa are distinguished only by subtle physical details, though their breeding biology differs dramatically: The Canadian and Appalachian tiger swallowtails are both spring-flying species, exhibiting only one generation per year, while our Eastern tigers crank out several generations, and can be found more or less continuously from early May until about the end of September. In any case, the Vineyard lies safely away from the zones in which P. glaucus bumps up against its sibling species; if you see a tiger swallow on the Island, rest assured that it is an Eastern.

One other oddity of P. glaucus is its ability to produce a so-called dark morph — a form that is mostly black rather than yellow. Dark tiger swallowtails can easily be confused with, say, spicebush swallowtail, which is normally and reliably black, though the tigers always show hints of their characteristic dark striping. The dark morph, for reasons I’ve never seen explained, occurs primarily farther South, and as far as I know has never been observed on the Vineyard. Still, it’s something to keep in mind, and would be a fun thing to document here.

Like all butterflies, tiger swallowtails are associated with particular plant species that serve as food for their larvae, or caterpillars. In the case of this species, these larval host plants are all trees. Of the known hosts, only one, black cherry, appears likely to be used on Martha’s Vineyard. I’m embarrassed to say, though, that I have never seen a tiger swallow caterpillar or an adult laying eggs, and so can’t confirm that with certainty. (The caterpillars resemble bird dropping when young, and mature into robust green critters with mock eye spots behind the head; they look kind of frightening, which is surely the point.) Whatever the host plant is, though, it must be common and widely distributed here, or tiger swallowtails could not be as prevalent as they are.

Adults of this species fly powerfully and range widely. I typically observe this species when it’s in flight, often quite high in the air, and moving briskly with a pattern of several wing flaps followed by a glide. Like some other butterflies that travel a lot, they evidently find roads to be convenient corridors to move along, and a high percentage of my sightings of this butterfly occur while I’m driving. But tiger swallowtails are also prodigious nectar hounds, feeding often and deeply on a wide range of flowers to power the butterfly’s energetic lifestyle.

I recently enjoyed a solid half-hour of observing a tiger swallow on our deck in Oak Bluffs. Dropping in suddenly from a high-level transit across our yard, the visitor zeroed in on several potted lantanas that were in full bloom. The butterfly fed enthusiastically, probing the tubular flowers with its long proboscis, sometimes hovering as it fed, and sometimes perching. After five minutes or so of feeding, it would fly to a nearby cedar tree and perch on a twig tip, hanging vertically and basking briefly in the late afternoon sun before resuming its meal.

While it fed, the butterfly was nearly unperturbable; I banged out scores of point-blank, frame-filling photographs, with the butterfly simply ignoring my bulk and the macro lens I kept aiming. This was, simply, a lot of fun, a reminder of how a good look at a butterfly can brighten your day.