Driving Birds Away

If you were a bobolink thinking about breeding, you would avoid laying your eggs within three-quarters of a mile of either side of a busy four-lane highway that runs by Thoreau’s Walden Pond in Concord, Massachusetts, or within a quarter mile of the through street connecting Lincoln to Concord. The 30,000-plus cars, trucks, and motorcycles speeding along Route 2 each weekday, and the 8,000 to 15,000 vehicles on the through street, make noise. That noise—not exhaust stink or the sight of speeding machines—apparently creates the broad avoidance zone on either side of the road, a wide swath of degraded habitat invisible to passing motorists, where certain birds don’t go or don’t breed.

SOURCE: R.T.T. Forman, B. Reineking, and A.M. Hersperger, "Road Traffic and Nearby Grassland Bird Patterns in a Suburbanizing Landscape, " Environmental Management 29 (2002): 782-800; and R.T.T. Forman et al., Road Ecology: Science and Solutions (Washington, DC: Island Press, 2003).

Graph by Stephen Anderson

Richard T.T. Forman and colleagues studied the impact of different-sized roads on the behavior of grassland birds in a 150-square-mile area along a 15-mile stretch of Route 2, just to the west-northwest of Cambridge, where Forman is professor of advanced environmental studies in the field of landscape ecology at the Harvard Graduate School of Design. This is a middle- to outer-suburban landscape with expanding residential areas in a landscape of forest, wetlands, ponds, streams, and open agricultural fields.

Many species of birds that thrive in open grasslands are now rare in Massachusetts, and some species—the northern harrier and the horned lark—no longer breed in any of the 84 patches of grassland in Forman’s study area. Using data that a team of birdwatchers gathered over five years, he concentrated on the bobolink and the eastern meadowlark. Though still present, these two species have been declining as breeders for decades—the same decades in which vehicular traffic has increased at a rate of 3 percent a year.

A quiet road traversed by no more than 8,000 vehicles a day had no effect on the presence or the regular breeding of his birds (see graph). But the greater traffic of a through street discouraged breeding, and the multilane highway drove the birds away entirely.

The edge species, our common backyard birds—the chickadee, let’s say— aren’t bothered by the racket of cars unless it is brutally loud; nor are egrets and herons, which one may spot in Cambridge intently fishing along the banks of the Charles River.

Forman doesn’t know why noise bothers grassland birds, but he hypothesizes that it places them in added danger. “When there are eggs on the nest and a cat shows up, or a snake or a hawk, the adult male or female makes an alarm click or call, and the adults freeze and so are not seen,” says Forman. “Those alarms are similarly critical when baby birds are fledged and on the ground. If the traffic noise is loud enough, the birds can’t hear the alarms.” Species with lower-pitched calls are hit the worst because traffic noise is low-pitched. (Work in progress reported to Forman suggests that where there is traffic noise, birds raise their voices and sing louder.)

Road ecologists such as Richard Forman advocate making passages for animals, over and under highways, that can help restore connectivity. This one has just been constructed under Route 2 near Walden Pond for the use of the fox, the coyote, the mink, the fisher, the raccoon, and other inconvenienced suburbanites. It is, says Forman, the first such underpass built in Massachusetts, the first of four planned, and one of very few in North America.

Courtesy of Richard Forman

Mitigating steps can lessen the roar: pave with a more-sound-absorbing surface; redesign tires, engines, and exhaust systems; reduce the proportion of trucks; build low, shrub-covered berms by the roadside; sink the road.

Forman is a pioneering landscape ecologist who became interested in what has recently come to be called road ecology when he realized that, although people knew a lot about the effect of nature on roads (potholes, for instance), they were largely blind to the effect of roads on nature. He is one of two lead authors of a field-establishing book, Road Ecology: Science and Solutions, written by 14 transportation specialists, hydrologists, and ecologists, half from academia and half from government. “It’s a solution-oriented book,” says Forman. “We tried to make it unavoidable by members of the transportation community.”

The network of public roads in the United States is a huge structure: four million miles of it, carrying a quarter of a billion vehicles. When one considers the avoidance zones surrounding many roads, Forman points out, the road network has an environmental impact on 20 percent of the total land surface of the country. Roads also create barriers to the movement of creatures great and small, from bears to salamanders. This reduction of landscape connectivity is the second big impact of the road system on wildlife. We have superimposed on our waving fields of grain and purple mountains a grid of interlocking roads that connect points for people but subdivide nature. We have built what Forman characterizes as a “megazoo.”     

Richard Forman e-mail address: rforman@gsd.harvard.edu