As the ferry I was on turned into the channel into Woods Hole the other day, a flock of about 30 crows, leaving the mainland, passed over the boat and headed across the sound, aimed roughly at Lambert’s Cove. Somewhere along the line, it became the season of the Vineyard’s remarkable commuting crows.
Though it surprises me each fall, the phenomenon is neither new nor obscure. For several decades at least, and possibly longer, large numbers of crows have aggregated into communal winter roosts on the Vineyard, flying to the mainland every morning, dispersing across the upper Cape (and perhaps beyond) to feed, and returning in late afternoon. But each year, things change a bit: the crows are more numerous, or less so; roost locations gain or lose favor (and crows); but each year, the migrations start subtly in the fall and end in spring the same way, starting as a trickle, swelling to a stream, and then fading again as warm weather takes hold.
For years, I lived under a main pathway for these birds on their daily commute from a roost in the pitch pine woodland at Trade Wind Fields Preserve (a.k.a. “the Dog Park”) in Oak Bluffs. At its peak, this roost hosted several thousand crows — a number that is not remarkable for the winter roosts of this gregarious species, but which still represents a lot of birds. I used to drive up onto East Chop on the occasional winter evening to watch them arrive, a stream of loosely formed groups, up high if the weather was calm, hugging the waves if a south wind pushed against them. And even from my house, a parade of crows was visible every morning and evening.
There is still a roost at Trade Wind, but it’s much smaller, at most a few hundred birds. And elsewhere on the Island, a roost formerly around Lake Tashmoo seems to have moved westward and grown steadily in numbers over recent years. Why? Who knows? You’d have to ask a crow.
They must be mainland crows, for the most part, since the total population of our multiple winter roosts far exceeds the numbers of crows one sees here in summer.
It’s generally assumed that crows dislike crossing large bodies of water. Reasonably so: they’re heavy birds, and a flight across a couple miles of ocean is energy-intensive and at least somewhat risky. So there must be a compelling reason for these flights, from a crow’s perspective.
The best explanation anyone has proposed is that they fly here to avoid nocturnal predation by great horned owls, a sworn enemy of the crow and a predator that is far less common on the Island than on the adjacent mainland. Great horned owls occur here now, but they don’t seem to be getting much traction: their numbers seem frozen at about three pairs, and it’s not clear whether these pioneers are successfully raising young.
But other changes are afoot. We have, for example, two species of crows in our region, and one of these, the fish crow, was long assumed to be absent from the Vineyard. Fish and American crows are hard to distinguish; the latter is larger on average, and the former has a thinner bill and slightly more pointed wing-tips. But birders rely on calls — the well-known “caw” for American crows, versus a much more nasal “cuh” for fish crows — to tell them apart.
However, young American crows often sound like fish crows, and I think it was Rob Culbert who first floated the then-heretical idea that commuting birds squawking like fish crows might actually be…fish crows. Subsequent observations, including some Christmas Bird Count reports by off-Island participants, firmed up Rob’s hypothesis, and it is now well-known that both species of crow occur here. Who knows where they will hang out this year? But last winter, fish crow flocks were reliably found near Five Corners and around SBS on State Road. So far no one has found nesting fish crows on the Vineyard. But they could be there, under the radar.
Highly social and highly intelligent, crows are among our most interesting birds. They communicate among themselves about food sources, dangers, and (obviously) travel plans. Their habits serve them well. Although crows suffer severely from West Nile virus when it is active (mortality among crows that get infected is said to be close to 100 percent), American crows remain common, and the fish crow has been steadily expanding its range in the Northeast for decades now.
Crows eat nearly anything, not much concerned with what people think: crows will ravage a corn crop one day, and the next, helpfully pick insect pests off the kale in the next field over. People have argued for centuries over whether crows are economically harmful or beneficial on balance, but I doubt the crows care. They just work at being crows.