Mourning cloaks

Butterflies are back — can spring be far behind?

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A mourning cloak butterfly. Photo by Matt Pelikan

The Bay State’s 2016 butterfly season got off pretty much on schedule this past weekend with a report of a mourning cloak from the Framingham area — the first butterfly I’ve heard of in the state this year.

I did some searching myself on Sunday, but came up empty; perhaps someone else on the Vineyard had more luck than I did. But it’s rare that Vineyard observers meet or beat the early date for this species on the mainland; at this time of year, ocean temperatures in the 30s tend to cap our warm days at anywhere from six to 12 degrees cooler than what interior Massachusetts experiences. (We make up for this in the fall, when the ocean, cooling more slowly than the land, often extends our season for several weeks after mainland butterflies have called it quits.)

Over nearly 40 years of looking for late-winter mourning cloaks, I’ve found the conditions that prod them into activity to be remarkably consistent. Two days of temperatures in the low to mid-50s generally does the job. The minimum air temperature for finding this species, though, varies depending on how bright the sun is. Typically, our mildest winter days also feather a thin overcast, muting the effect of the sun, but on the rare late winter day when unseasonable warmth combines with pure blue skies, I think you might find these butterflies on a day in the upper 40s.

The presence of any butterfly at this point in the season always comes as a bit of a shock to my system; even if I’m looking specifically for mourning cloaks, the first one I spot always has a jarring effect, like a leaf that is defying gravity. The fact is that nearly all of our butterflies are solidly dormant now, overwintering as eggs, caterpillars, or pupae. The mourning cloak is one on a very short list of species that overwinters as a fully mature adult. Eastern commas and Compton tortoiseshells are other butterflies exhibiting this odd life cycle, but both are vanishingly rare on the Vineyard, apparently arriving here just as the occasional summertime vagrant from the mainland.

In any event, the elegant mourning cloak is invariably on the wing here by the third week of March, and in years when I’ve been able to spend a lot of time in the field in late winter, I’d say I find a mourning cloak in February about half the time. There is at least one January record for this species on the Vineyard, and several more such records for mainland Massachusetts. Earlier this year, I received an interesting report from Nancy Rogers of an indoor mourning cloak on the Vineyard. A little investigation revealed that the appearance of the butterfly coincided with the arrival indoors of a load of firewood, and the butterfly was undoubtedly overwintering in Nancy’s woodpile.

This is by no means the first such story I’ve heard, and indeed I think that woodpiles offer a structure that mourning cloaks find nearly ideal for overwintering. Under purely natural conditions, they hide themselves behind a flake of bark on a log or tree trunk, fill their cells with a natural form of antifreeze, and wait until conditions feel right to emerge again.

Once they’re on the wing, mourning cloaks show a penchant for using the same basking sites year after year. Since the species lives less than a full year, it is different individuals occupying favored sites each year, and I assume certain places feature a microclimate that is particularly attractive to this species. The spur trail leading to the summit of Prospect Hill, at Menemsha Hills Reservation in Chilmark, is one such place, and I’ve found my first mourning cloak of the year here in several of my 18 springs on the Vineyard.

Mourning cloaks are unmistakable butterflies, somewhat smaller than a monarch, deep brown (nearly black) with a yellow border on the wings outside a row of exquisite blue dots. You’ll typically find this butterfly basking in the sun on bare ground or, sometimes, a rock. If disturbed, a mourning cloak usually circles back eventually to its original perch. The species rarely drinks nectar (not that many flowers are in bloom now anyway), but they will drink sap from a wounded tree, and they show a regrettable fondness for ingesting liquid and minerals from animal scat or urine.

Once awake, mourning cloaks mate, and then the females lay eggs on willow shrubs (the food plant for caterpillars of this species). Mourning cloaks do not breed successfully here very often, though, and there are relatively few records of summertime adults or of caterpillars (Nancy Weaver found some a few years back at Polly Hill Arboretum). Mourning cloaks are somewhat migratory, however, and decent numbers of them pass across the Vineyard in September and October. A few of these find a site to their liking, hunker down, and wait for spring.