Wild Side: Brown thrashers have become rare here

Our habitat does not seem to be the issue.

Brown thrashers have been declining in numbers on-Island and statewide. —Paul Crook

A recent report in a Vineyard birdwatching Facebook group called to mind a species I hardly ever think of these days: the brown thrasher. It’s not that I don’t like them; indeed, thrashers rank among my favorite songbirds. It’s just that this bird, once widespread and numerous, has quietly, steadily slid downward in abundance to the point that it has become a rarity on Martha’s Vineyard.

Thrashers are large for songbirds, averaging a bit longer than blue jays. Long-billed and long-tailed, they are a distinctive rusty brown above; their white underparts are heavily marked with fine black streaks. In behavioral terms, “thrasher” is an apt name, describing this bird’s energetic feeding habits: Typically they feed on the ground, kicking or throwing aside the leaf litter in pursuit of insects. Like their close relatives the catbird and the mockingbird, thrashers are fond of fruit and berries, especially in winter.

Again like its relatives, the thrasher is musically quite accomplished. Males, usually singing from an elevated perch, give a sustained, inventive song of constantly varied phrases, similar to the song of a mockingbird. Conveniently, while mockingbirds typically repeat each phrase multiple times, thrashers sing each phrase twice, almost invariably. This feature makes the thrasher’s song quite easy to recognize. Thrashers also give a harsh, distinctive call note that sounds like “chack!”

As the 20th century was poised to start, brown thrashers abounded on Martha’s Vineyard, and indeed in much of Massachusetts. According to the 1959 volume “Birds of Martha’s Vineyard,” by Guy Emerson and the legendary Ludlow Griscom, ornithologist William Brewster tallied 400 thrashers on the Vineyard on June 28, 1890. That’s a figure that I can’t help suspecting somehow acquired an extra zero. But even 40 of something in a single day, dispersed across breeding territories, suggests true abundance.

When I arrived on the Vineyard in 1997, brown thrashers were uncommon, but by no means hard to find. In my early years on the Island, for example, I knew of five places within Correllus State Forest where this species nested annually. By 2007, though, when Soo Whiting and Barbara Pesch published “Vineyard Birds II,” they accurately described the species as a “declining summer resident in small numbers, and uncommon transient” on our little sandbar.

At this point, I’m sorry to say, I don’t know of any reliable locations at all for nesting thrashers. The species continues to turn up sparingly as a fall and (even more sparingly) spring migrant. And thrashers, which are tough, resourceful birds, often linger into or even through the winter in very small numbers. The species remains more or less annual on the Island’s annual Christmas Bird Count, though it has become rare for more than one or two to be found. This is not, at any season, a species that you go out expecting to see.

A similar decline has been evident regionally. The two statewide Breeding Bird Atlas projects that Massachusetts Audubon has coordinated — one conducted in the early 1970s, the other from 2007 through 2011 — paint a depressing picture of steep decline throughout the state.

The off-Island decline seems fairly easy to explain. Breeding thrashers prefer scrubby habitat, either shrubland or early second-growth woodland. From the mid-19th century through much of the 20th, vast tracts of abandoned farmland provided exactly this type of habitat across the region. Suburban development, combined with the natural succession of ex-farmland back into forest, has greatly reduced the amount of this type of habitat, and the thrasher (along with some other thicket-loving birds, such as the towhee) has accordingly declined. The relative abundance of thrashers that I recall from my childhood in the Boston suburbs, then, was a relatively new and entirely anthropogenic phenomenon: A rather specialized bird species, probably not very common aboriginally, was the temporary beneficiary of regional-scale patterns of land use by humans. In that sense, the recent statewide decline of this bird could be viewed as not so much a conservation problem as a return to pre-European-settlement conditions.

But what about on the Vineyard? The thrasher’s preferred habitat on-Island is scrub oak thicket, and that’s a habitat type that conservation-oriented land management has sought to maintain or even expand. Given the extent of prime scrub oak in Correllus State Forest, I can think of no reasonable local explanation for the apparent disappearance of breeding thrashers from the forest.

If the local decline can’t be explained by changes in breeding habitat, it seems to me that the problem lies in the areas where our breeding thrashers winter (or would winter, if there were any of them left). I don’t know for sure, but I expect our breeding thrashers simply drifted south along the coast in late fall and early winter. Perhaps it’s loss of wintering habitat from, say, New Jersey down to the Carolinas that is to blame.

Even if its decline is in some sense a return to normal conditions, I’m still sorry to see the dwindling of the brown thrasher. These are beautiful, vocal, and charismatic birds, and I miss the pleasure of encountering them regularly.