September is the peak of migration, not just for birds but for bats, butterflies, and dragonflies as well. We tend to think of fall migration as mainly a southward movement, and that makes sense. Moving toward warmer weather, organisms can extend their lives, lay more eggs, and, in the case of vertebrates, survive until the following spring and summer.
Oddly, though, for some species of butterflies, fall migration points north. This behavior by cold-blooded animals seems, at first glance, much harder to explain. As the weather cools and the days get shorter, why would a butterfly want to head deliberately toward even cooler air and shorter days? But if you think about migration from the perspective of a species rather than an individual, the pattern starts to make sense.
For one thing, the late summer movements of these butterflies may not be as deliberately northward as it appears. Perhaps these species just have a general instinct to disperse from their original locations? Fanning out in all directions, most individuals would end up in areas already occupied by their species, and so be overlooked. It’s only the ones on a northward track that get noticed, as they turn up in areas previously devoid of their species.
But a northward migration still makes sense even if it’s the main direction of travel. Yes, for most of the individuals in such a migration, the trip is one-way with a dead end. Even if they find a suitable host plant to lay their eggs on, the resulting offspring may be doomed by the winter to come, or the short growing season that follows.
But things change, and the long-term survival of any species (that includes our own) depends on building in a certain capacity for flexible response to change. Most notably, climate change has produced a novel and rapidly developing set of circumstances for butterflies. Areas previously inhospitable to many southern species have grown marginal, and then downright comfortable as winters have grown, on average, milder, and growing seasons have gotten long enough for butterflies to cram in two or three generations in a season.
Real-life examples of northward range expansion are easy to find, with some instances taking place over just a few decades — time frames short enough for individual observers to watch it happen.
The sachem (Atalopedes campestris) furnishes one example. A southern species long noted for a tendency toward northward migration in late summer, the sachem was first recorded in the Bay State in 1995 (on Martha’s Vineyard, as it happens). While the species wasn’t found on the Vineyard again until 2005, it almost instantly became annual in Massachusetts. And by about 2010, it appeared established in some areas, including here, exhibiting multiple generations in a year and turning up in numbers too large to plausibly reflect just migrants.
Today the sachem is well established across coastal southern New England, with records north and inland no longer surprising. Look for the northward march of this small butterfly to continue: Its preferred host plant in our region, crabgrass, is plentiful, and the sachem flourishes in heavily settled suburbs and other developed areas. Careful attention to the flowers in your Vineyard yard will likely reveal the presence of this species.
A more recent Island arrival, but one that may be settling in even more quickly, is the red-banded hairstreak. This elegant southern species began turning up in southern New England as a late-season migrant in the mid-1990s. It was late to arrive on Martha’s Vineyard, with the first record (presumably a migrant) in September 2016. The species rapidly became annual here, and now, populations appear to be established in at least two locations: West Chop, and along Lambert’s Cove Road.
Again, the food plant is a common one (sumac). All this species needed was a slight moderation in average winter temperatures to become viable at our latitude.
Finally, 30 years ago, the Zabulon skipper’s range barely touched the Bay State. Its range map in my 1999 butterfly field guide shows this species barely making it to southernmost New England, and in Massachusetts, it was restricted to a small population in the lower Connecticut River valley.
Around 2008, the species broke out into wider distribution. About 10 years ago, it made its way to the Vineyard. Now, it’s well established here — I see it almost daily during its two population peaks each year — and on the mainland, it can be one of the most common butterflies you encounter in eastern Massachusetts and the Connecticut River Valley. Its range, currently extending into southernmost New Hampshire, appears to be steadily expanding.
Migration, then, is not just a behavior aimed at individual survival. It has implications for entire species, as well, facilitating exploration for new areas to inhabit and affording the flexibility for a species to respond to, or even exploit, changing conditions. Viewed from that perspective, the counterintuitive northward movement in fall makes perfect sense.