Wild Side : Bird species come and go on Martha's Vineyard over time
Martha's Vineyard Times File Photo
A phrase like "the bird life of Martha's Vineyard" projects a certain grandeur and sense of permanence. But don't be fooled. The mix of species here has never been stable, with nesting species fading away and new breeders replacing them for the entire history of the Island.
While other factors may play a role, the disappearance of a breeding bird often reflects the loss of suitable nesting habitat for that species. Such declines in bird populations often occur gradually enough so people don't notice them. But while a decline of, say, three percent per year doesn't really figure when you compare this year to last, after a few decades, you suddenly realize a once-common bird is gone.
Such was the case with several open-country species on the Vineyard, displaced as sandplain grassland and sheep pasture either succumbed to development or (with cycles of fire or grazing interrupted) grew up into oak woodland. Upland sandpiper, short-eared owl, and vesper sparrow no longer nest here (though we still see them as migrants); only the grasshopper sparrow hangs on, the remnant of its once-large population now down to a precarious couple of pairs on up-Island grassland.
Areas given over to development lose much of their bird diversity. But the good news is that as long as changing habitat remains natural, other birds will discover it. Vesper sparrows may no longer sing on the hills of Chilmark. But the oak woodland that has regrown there supports forest species like the wood thrush that must have been rare here during the sheep-farming days.
Forces other than habitat change may figure in the gain or loss of breeding species, though; here are the stories of three recent arrivals.
We might start with the Cooper's hawk, a mid-sized raptor with a fondness for birds as prey. This taste extends to birds the size of chickens, and this hawk, for many decades, was relentlessly gunned down as vermin because it was suspected (and often guilty) of preying on poultry.
Like other raptors, Cooper's hawk also suffered from poisoning by DDT and other highly toxic insecticides. But with these harmful practices stopped, most species of hawks began rebounding quickly; in recent decades, Cooper's hawks have once again become common nesting birds in southern New England, with the first pairs colonizing the Vineyard in the mid- or late 1990s.
In contrast, the overall numbers of the indigo bunting have remained fairly stable in the region, so the establishment of a breeding population of this finch on the Vineyard must reflect something other than growing numbers. In a way, the absence of this bird as a breeder here was mysterious, since they're common in New England and have always occurred regularly as spring migrants. Allan Keith has proposed that potential nesting indigos were long deterred by our late springs: between our marine climate and the local dominance of late-leafing oaks, Vineyard vegetation was still bare and unappealing during the relatively early migration window of this beautiful bird.
But springs have been getting earlier, as a result of climate change. After a storm in March 2004, dozens were reported here — an exceptionally large early-season "fallout" of this species on the Vineyard. A few pairs remained here and nested successfully. While not exactly common, the indigo bunting is no longer a surprising sight here in summer, and this species, which prefers young second-growth, found additional habitat in areas of woodland killed during the recent caterpillar outbreak. The past few seasons, I've known the location of about a half-dozen territorial males, and the entire Island population is probably several times that size.
While the establishment of indigo buntings can be traced by one unusual migration event, nobody knows how tufted titmice got here. Probably because titmice dislike flying across water, the Vineyard was passed by as this species began expanding its range northward in the 1950s and '60s, the beneficiary of feeders and habitat structure in suburbia. But in 1997, the presence of a reproductively active titmouse was proven by the discovery of a chickadee-titmouse hybrid at Seven Gates. A single-species pair of titmice was found soon after in the same area, and the distribution of this close relative of the chickadee rapidly expanded to cover most of the Island. Once unheard of as a nester here, the tufted titmouse is now quite common.
Never static, the bird life of our Island is in constant flux. Blue-winged warblers are growing more common; the chimney swift, alas, is in decline. Perhaps the hooded warbler found singing this past week at Waskosim's Rock will attract a mate and turn out to be the patriarch of a new population.
Habitats change; ranges shift; even the climate around us alters with time. And birds, resourceful and infinitely mobile animals that they are, find the resources that result.