The depth of winter is hardly prime birding season on the Vineyard, but this season is not without its consolations. If you spend most of your life on the tundra, on the coast of Labrador, or in a boreal spruce forest, winter in southern New England doesn’t seem so bad. Accordingly, for some northern birds, the Island is a popular winter destination.
Some of these birds occur regularly and in large numbers — for example, common eider and the three scoter species, which bob complacently all winter on near-freezing water. Other species are generally around, but scarce or hard to find. Snow buntings, for instance, favor the austerity of our outer beaches for their winter habitat, and I suspect there is a flock around somewhere during most of the winter, although finding it can be a challenge.
Still other birds turn up here unpredictably in winter, absent sometimes for many years in a row before conditions to the north of us bring them to the Vineyard. Snowy owls, for example, happily spend most winters well to the north of us. But in some years, a shortage of prey or an unusually dense population of the owls themselves drives these birds south, sometimes to our shores.
This winter has featured notable numbers of one such specialty, the so-called winter finches, on the Island and indeed throughout the region. Pine siskins, common redpolls, evening grosbeaks, and most recently, white-winged crossbills have brightened the season, along with red-breasted nuthatches (unrelated to the other species, but an honorary finch for purposes of winter visitation). All these birds share a northern breeding range, an association with boreal habitats, and a peculiar tendency to head south in massive waves during some (but not all) winters.
Termed “irruptions” in birder-speak, these unpredictable movements are not strictly migrations, since that term implies regularity in timing and destination. Instead, a finch irruption (which may involve one, several, or all of these species, and may last for weeks or months) appears to be triggered by a collapse in food supplies (such as a poor year for spruce cones in the northland).
In response, flocks of hungry finches wander southward, seeking alternative resources. These journeys can be impressive; red-breasted nuthatches, for example, are typically confined to the northernmost states or higher elevations in the Rockies and Appalachians – but in irruption years, this species has been recorded all the way to the Gulf Coast and indeed beyond, with a number of records from oil rigs in the Gulf of Mexico!
One consequence of the nature of irruptions is that these birds may disappear almost as soon as they arrive. Moreover, my guess is that irrupting finches tend to arrive on the Vineyard by accident rather than design: we tend to get smaller numbers than the mainland does. Often, Vineyard birders encounter individuals or small groups rather than the large flocks that mainlanders sometimes find. A Christmas Bird Count a few years ago, for example, produced a notable (for the Vineyard) count of several hundred common redpolls — but these birds were seen at Gay Head, at dawn, leaving the Island behind.
A recent wave of white-winged crossbills, then, fits the Vineyard pattern: an abrupt scattering of reports, the first from an Indian Hill feeder, spaced over a few days and spanning most of the Island, followed by an equally abrupt disappearance of this species (or so it appeared as this column was being written). Siskins, evening grosbeaks, and redpolls exhibited a similar pattern earlier in the winter.
Crossbills are notoriously fond of conifer seeds; indeed, their unique bills evolved specifically for exploiting cones. The tips of the mandibles cross; when a crossbill closes its beak, the tips of the mandibles actually move apart. These birds, then, can use the powerful muscles that close their beaks to jack open cones, exposing the seeds (and sometimes dormant insects) concealed inside. Crossbills readily eat other kinds of seeds (as far as I know, all the recent reports came from feeders, with thistle seed the food of choice). But irrupting crossbills probably seek areas rich in cone-bearing evergreens.
On Sunday, then, I birded an extensive patch of spruces in the state forest, east of Barnes Road, hoping to find some crossbills. I knew I was cooked before I even got to the spruces: scanning with binoculars from a few hundred yards away, I could see that the Vineyard cone crop failed in 2010 — the trees were coneless, and being coneless, were also birdless. Two hours of searching produced two chickadees, two red-tailed hawks, and the calls of a few distant crows.
So it goes, though, with winter birding. Harsh conditions and limited food supplies govern the behavior of irruptive birds, which have no use for habitat that doesn’t offer a wealth of resources. I’m sure crossbills checked the state forest spruces — you could hardly miss this swath of green from the air. But, finding nothing, the birds continued their restless journey, perhaps elsewhere on the Vineyard, perhaps far beyond our shores.