Wild Side : Tree swallows migrate over Martha's Vineyard
Fall migration brings rare birds to the Vineyard, which makes Island birders happy. But rarities are rare. More predictable, and in some ways more interesting, is the spectacle of sheer numbers that certain species provide in passing through our region.
One of the most reliable of the Vineyard's avian spectacles is the autumnal passage of clouds of tree swallows. Indeed, any spot along the Massachusetts coast may host tree swallows through the fall, with Plum Island, various points on the outer Cape, the shoreline of Chappy, and the Vineyard's south shore among traditional places for them to congregate. It's a thoroughly predictable phenomenon, known to ornithologists for many decades, clearly a normal part of the behavior of this bird.
Often, migrant swallows occur as dispersed individuals hunting flying insects; you may see only a few at a time, which isn't very impressive until you realize that swallows at the same density probably extend all along the Island's southern shore. Other times you'll encounter flocks, large or small, perched on wires, shrubs, or reed beds. Along with robins and red-winged blackbirds, tree swallows like to roost in stands of Phragmites such as the ones around the eastern end of Chilmark Pond. And sometimes you'll encounter thousands of swallows flying in rolling, cloudlike flocks. The precise timing and location of the biggest flocks is hard to predict, but you can be sure they will appear. Perhaps the largest flock I've ever seen on the Vineyard was a group I estimated at 40,000 birds at Cape Poge in mid-September, and flocks up to ten times that size have been noted. Numbers will decline through the fall, though a few swallows often linger to be tallied on the Vineyard's Christmas Bird Count.
As numbers like that suggest, the tree swallow is a common bird across a large geographical range: this species nests in a broad band that extends across much of the United States and southern Canada. One estimate places the total population around 20 million, and in some parts of the range, at least, numbers are increasing. The Breeding Bird Survey program, run by the U.S. Fish and Wildlife Department, shows that the species is most abundant in eastern New York state and southern New England. Recoveries of banded tree swallows prove that birds from a substantial portion of the tree swallow's core range — the eastern Great Lakes, New England, and southeastern Canada — generally head for the coast and move south along the shore, their instincts guiding them on a route that eventually takes them to Florida and the West Indies for the winter.
The coastal route is used by many migratory birds, especially young of the year, for their fall passage. And for good reason: An oceanic shoreline provides an infallible navigational aid for birds that have the instincts to follow it. Moreover, the proximity of the ocean moderates autumnal cooling, meaning that conditions stay milder than inland, and insects stay on the wing later into the season. And shrub species typical of coastal thickets (winterberry, viburnum, and especially bayberry) produce berries that remain on the plant into the winter. Tree swallows are one of relatively few bird species capable of digesting the waxy fruit of the bayberry; the fruit provides a highly concentrated energy source for the birds, who return the favor by carrying bayberry seeds around in their innards and "planting" them in their droppings.
Tree swallows, of course, don't just visit the Vineyard in the fall; this is a familiar breeding bird here, one of the first migratory species to return to the Island in spring (most years, I've found a few small groups of swallows before March is gone). Spring arrivals, especially males, are tidy-looking birds, iridescent blue-green on the back and pristine white underneath. Females, especially young ones, are somewhat duller, and the majority of our fall transients are first-year birds, dull brown above.
Cavity nesters, tree swallows will breed in either natural holes in trees and snags, or in nest boxes, laying relatively large clutches of about six eggs. By nesting in cavities, birds afford themselves a measure of protection against both predators and bad weather. Even so, tree swallows are subject to disastrous breeding seasons when rainy weather makes insects hard to find, causing massive die-offs of swallows due to starvation or hypothermia. Studies suggest that 80 percent of fledglings fail to survive their first winter, and there are relatively few records of banded birds in the wild living more than three or four years.
But as the reliable appearance of huge flocks makes clear, the lifestyle the tree swallow has evolved generally works well. Prolific breeding, adaptability, a wide geographic range, and a shrewdly chosen migration strategy help the species overcome adversity and produces a stunning fall spectacle for Island observers.