Wild Side: ‘Overshoot’ events

Our April arrivals may change the Island bird scene.

An adult male indigo bunting. — Ken Magnuson

Autumn is typically the peak season for Vineyard birding, as migrants stream south along the coast, taking advantage of ocean-moderated temperatures and resource-rich coastal thickets. Spring migration, in contrast, often means just the return of our breeding birds, with the occasional, unremarkable vagrant thrown in.

But some years, weather conditions conspire to reverse the model, turning our usual lackluster spring migrations into exciting events. Sometimes a strong southwesterly blow follows a long period of entrenched northerly winds, splashing a pulse of delayed migrants onto our shores. Other times, the circulation of a coastal storm picks up songbirds in the Southeast and flings them our way over the open ocean.

In any case, the result is striking: The abrupt appearance of large numbers of migrants, often including “overshoots” — species that should not yet have arrived at our latitude, or even species that shouldn’t occur this far north at all. Island birders go nuts.

Late April of this year featured such an event. With my usual impeccable timing, I was traveling overseas and missed the entire event. But the Island’s birding Facebook group boiled over with reports indicative of the best spring birding in many years. Ken Magnuson started things off on April 21 by photographing a black-whiskered vireo, a Caribbean bird never before recorded north of the Carolinas, and one of the best birds ever found on the Vineyard. Other reports of vagrants and overshoots rapidly piled up: summer and scarlet tanagers; blue and rose-breasted grosbeaks; yellow-throated, prothonotary, and hooded warblers; painted bunting; and more.

As is usual with overshoot events, a signature species of this fallout was the indigo bunting. Sparrow-size adult males are stunningly beautiful, a crystalline blue all over. Females are soft brown, nearly featureless birds. Indigo buntings are easy to find here during fall migration (mainly drab immatures, which resemble adult females), and we get a few normal transients every spring. But for some reason, indigo buntings also figure in every fallout, often resulting in a strong pulse of arrivals weeks before migrants on their usual schedule arrive here.

Despite this pattern, indigo bunting was for years unknown as a breeding bird on the Vineyard. I like Allan Keith’s explanation: Arriving in early May, when Vineyard vegetation may be weeks behind that of mainland locations in leafing out, migrant buntings take one look around, say something rude about the Island’s bleakness, and move on to someplace that has leaves.

I think it was in 2003 that things changed. The Island saw a massive influx of indigo buntings, perhaps even more striking than this April’s. I remember the late, great Vern Laux telling me of an up-Island field with 60 of them in it — even allowing for Lauxian hyperbole, clearly a startling sight on the Vineyard.

It was either that year or the next when this species was first confirmed breeding on the Vineyard — a pair, if I remember correctly, at Waskosim’s Rock Reservation in Chilmark. Reports of other singing males suggested other breeding sites. Clearly, some birds in that original influx remained here to nest, and they or their offspring continued return in subsequent years.

In 2005, indigo buntings first turned up on my Breeding Bird Survey (BBS) route. Run annually on behalf of the U.S. Geological Survey, this route consists of 50 fixed points along a 25-mile route, winding from Lobsterville to the West Tisbury Road, with each point the site of a three-minute bird count. You can’t be sure you’ll find every species that nests here, but this protocol samples enough real estate to give a good sense of our breeding birds and their relative abundance. (Or at least their relative detectability.)

For the next few years, I recorded two or three indigo buntings each time I ran the route, suggesting that the species was a well-established, fairly common part of our breeding avifauna. And each summer, I’d encounter a half-dozen or so other breeding pairs (surely only a fraction of the Island’s actual population) at various points around the Island.

But in 2011, I failed to record an indigo bunting on the BBS route, and while one pair of buntings hung in for a year or two near the Tashmoo Overlook, by 2015, the species was gone as a Vineyard breeder, as far as I could tell.

Until 2019? That, of course, remains to be seen. But you can be sure I will be looking. April’s fallout was the first event in years that produced reports of groups of indigo buntings, and Internet reports suggested that singletons of this species were briefly downright easy to find on the Island. The pattern mimics that of the early 2000s, raising the possibility that a critical mass of these birds has hit our shores.

Regardless of what happens, the history of indigo buntings on the Vineyard highlights the dynamic nature of our bird life. Some years we get hardly any; some years it rains buntings; some years they’re a common breeder; some years they don’t breed at all. Things always change, and it’s that simple fact that keeps birders heading out the door.