July is hairstreak month on the Vineyard. Now, that’s a remark that needs some explaining, partly because many people have no idea what a hairstreak is, and partly because even among people who do, it’s an imprecise term.
To keep things simple, the hairstreaks are a group of delicate butterflies, each wing about the size of dime, gray (with a few exceptions) overall but equipped with intricate stripes and patches of color on their wings. The “hair” in their name refers to tiny, thread-like tails extending from the margin of their hind wings, usually adjacent to patches of color. Characteristically, hairstreaks perch with their wings raised over their backs, rubbing them together. The combination of tails and patches is thought to resemble an eyes and antennae, an adaptation to make it harder for a predator to know which end is the front of the butterfly.
If one took a broad definition of “hairstreak,” the Island hosts about a dozen species. That number would include the elfins, four species of brown, early-season butterflies, with stumpy tails instead of hairs, because taxonomists put them in a genus with some less questionable hairstreaks. Less debatable members of the group would include the gorgeous juniper hairstreak, green with white striping, which flies in May and then again in late July. And it would include two gray hairstreaks that exhibit three flight periods per year, in May, July, and September — the gray and the rare white-M hairstreak.
But it is the genus Satyrium, represented by four, perhaps five species here, and the genus Fixsenia, with just one member here and a rare one at that, that constitute our roster of true July hairstreaks. With just one generation a year, these species appear as adults in the last days of June or the first ones of July, flourish for a few weeks, and then are gone until the next year.
Zealously sought by butterfly enthusiasts, hairstreaks generally are both challenging and rewarding for the observer. The rewards come from the sheer beauty of these insects. On a casual glimpse, they may appear dull gray and nothing more. But observed closely — and with care, you can get within inches of a comfortably perched one — they are simply gorgeous animals, ornately marked.
The challenges, though, are many. Some species are almost always scarce, and in some years, members of this group are nearly impossible to find. They are tiny and hard to spot. And their flight is erratic and incredibly rapid; were it not for a happy proclivity to return to the same perch, a hairstreak in flight would be gone for good most of the time.
But like all butterflies, there are tricks that make observation easier. For one thing, being highly active insects, hairstreaks are all avid drinkers of nectar — that is, of sugary liquid produced by flowers to attract pollinators such as butterflies. Hands down, the easiest way to find July hairstreaks is by checking milkweed blossoms for butterflies engaged in nectaring.
The social habits of hairstreaks also work for the observer’s benefit. They are fairly social insects; if you find one, others are likely around. And the males often perch on tips of leaves, looking gorgeous and exuding pheromones with the goal of attracting a female, and the silhouette of a perched hairstreak is tiny but sometimes surprisingly obvious.
Edwards’, banded, striped, and oak hairstreaks are all enjoying a banner season this year. (Coral hairstreaks, named for a reddish cast to their ground color, have been curiously absent so far, but they are usually the latest species to fly, and may simply not have begun their season yet.) The caterpillars of all these species feed on hardwoods of various kinds, so the habitat to look for them is easy to determine: fields or clearings in or near deciduous woods and rich in milkweed or other nectar sources. I doubt your eyes will work as well as my experienced ones, but in mid-July, I can often find more than 100 hairstreaks in a day.
The star of this year’s show has been the oak hairstreak, which as its name suggests associates with oaks. You’d think they’d be abundant on the Vineyard, which is so dominated by oaks, but think again. This is among the rarest of our butterflies, and I miss it altogether in more years than I see it. It is rare enough to be listed as a Species of Special Concern under the Massachusetts Endangered Species Act.
But along with other hairstreaks, the oak hairstreak seems to hit random peaks of abundance every few years, presumably when conditions over the past year have suited it. Adults are relatively plentiful for a few weeks — I’ve found about 10, at six different locations, this year — presumably dispersing and laying eggs in many corners of the Island, and then for the next few years the species fades into the background again. So this bountiful season has been one of delight for me.
