Sound science

by Mary Woodsen

Sometimes finding a nest was as easy as stepping behind a pinon pine: Suddenly a bird winged in, nesting material in its beak, and there you had it. Sometimes it meant tirelessly tracking a bird for two days, finally settling under a juniper at the center of its territory to wait it out -- and having your chief rival on the field crew point out that, hey, dude, the nest was right above your head all along.

The competition could be fierce. A nest might be worth five bucks, or maybe an ice cream cone, after the heat came on and the crew piled out of ecologist Clint Francis' dusty pickup truck back at headquarters in Durango, Colo. Mostly, though, it was good for bragging rights.

But for the lanky, brown-haired Francis, who began his study in 2005 and published the results in 2009, the payoff has always been the data. In a world growing steadily noisier, his groundbreaking research, done in the Bureau of Land Management's Rattlesnake Canyon Wildlife Area in northern New Mexico, offers the clearest proof yet of the impact industrial grade noise has on natural communities of birds.

A vast and ancient landscape of arroyos and oak flats, the mesas surrounding Rattlesnake Canyon are pockmarked with 1,500 natural gas well pads and crisscrossed with red clay roads. At least half those well pads are outfitted with humungous compressors continually pushing gas through feeder lines -- and radiating noise in every direction. Ecologists have worried for decades about how sweeping landscape-level changes affect the odds for wildlife. Yet noise can fragment habitat as surely as the physical fact of clear-cuts or interstate highways. And noise, Francis notes, is constantly growing in loudness and ubiquity.

During the past 40 years, the nation's population has grown by a third while road traffic nearly tripled. Eighty-three percent of the continental U.S. is now less than two-thirds of a mile from a road. Since the early 1980s, air traffic has grown by a factor of three; it's hard to find places free of high-altitude noise. But only in the past 15 or 20 years -- about the time those gas wells began going in -- have researchers considered what noise fragmentation looks like on the ground. And what better study subject than birds?

Among animals, birds stand out for their profound reliance on songs and calls. Songs define nesting territories, while calls keep birds apprised of friend and foe -- messages critical for survival and reproduction.

Although other studies have shown a link between, say, traffic noise and declining bird densities, none were able to control for a slew of unrelated variables. Most have looked at bird communities near highways -- "but  the  impact  of  traffic

noise is complex," says behavioral biologist Hans Slabbekoorn at Leiden University in The Netherlands. "There are so many confounding factors, such as chemical pollution, being unable to hear birds over traffic noise, and the visual disturbance of moving traffic."

The beauty of Francis' study sites is the scope they lent for a truly natural experiment, meaning he didn't have to add or subtract anything for his experiment to work. All he had to do was watch. His "controls" (sites with quiet soundscapes) and "variables" (sites with noisy soundscapes) were already in place. Each site shared the same basic plants, topography, weather and predators. Each site is populated only by birds native to the West.

The difference? Quiet sites had the acoustic landscape the birds evolved with: the sigh and rustle of wind, the chatter and chirping of chipmunks and cicadas, the yip-yip of coyotes or foxes. The noisy sites were blanketed day and night with an unrelenting din.

Francis' work had its origins in a Bureau of Land Management survey done in 2000 to see whether noisy compressors meant fewer bird species (and individual birds) in nearby forests during the breeding season. The short answer was no. It even suggested that some species were more common in noisy sites. Though the BLM survey team wasn't looking for nests, they couldn't help but notice house finch nests on well-pad equipment next to compressors. These adaptable natives are endemic in the West and expanding their range. And when the survey team turned the compressors off, they saw no indication that birds changed their behavior.

Still, BLM wildlife biologist John Hansen wondered if there might be more to it. He knew that their mere presence doesn't mean birds are establishing nesting territories or caring for young. Was it possible that noise had an impact on reproduction and survival? Hansen called Catherine Ortega, then a biology professor at Fort Lewis College in nearby Durango. Ortega in turn called Francis, at the time doing fieldwork on Mexican spotted owls and willow flycatchers.

Francis and Ortega set up a project to address Hansen's question, following the same basic game plan as the BLM survey. Field crews visited 400- by 60-meter transects laid out at each site. (Each transect is a narrow, randomly chosen study area; it helps keep the data honest.) They rotated among sites -- if a field crew visited one site for the dawn chorus, the next time they'd get to it later in the day. A gas-company rep met crews at each well pad, turning off the compressors for two hours so they could cue into the birds.

But Francis and the field crews did more than just count birds. They searched for nests. It was silent, patient work, crouching in the shadows with their binoculars, watching birds flit from branch to branch. Some nests, camouflaged with strips of juniper bark at the juncture of trunk and branch, looked like a burl, nothing more. Others lay tucked among dense stands of Gambel oak.

Once the crews found nests, they monitored them. They tracked how loud compressor noise was at each and noted whether any chicks survived long enough to fly from the nest. As Francis and Ortega had predicted, the more noise, the fewer species actually built nests and reared young. While 32 species, including black-headed grosbeaks, green-tailed towhees and Cassin's kingbird, nested in quiet sites, only 21 nested in sites near compressors. Of those that showed some tolerance for noise, many -- gray vireos, spotted towhees and gray flycatchers, for instance -- nested farther from well pads at noisy sites than they did at the quiet controls. Yet even so there were surprises.

For one, the number of nests was virtually the same across both types of sites. "And some species seemed to prefer noise," Francis says. Finding 94 percent of house finches nesting near compressors was a bit of a shock, even though the BLM survey had shown they were common on noisy sites. And talk about surprises in small packages -- 92 percent of black-chinned hummingbirds set up shop amid the noise.

The biggest surprise: Contrary to expectations, the birds that reared the most young nested closest to compressors, even though they most likely could not hear predators coming.

Two years into the project, Francis set up artificial nests baited with quail eggs to find out why. Motion-triggered cameras near the fake nests identified the principal predator -- the western scrub jay. But the jays couldn't hack the noise. Nearly twice as many foraged in quiet sites as noisy ones. When the jays did raid nests in noisy sites, they hit the nests farthest from the compressors. And no other predator took their place.

How did the finches and hummers cope with the noise? "House finches are known for being able to adjust the pitch of their songs up or down, depending on what kind of background noise they're competing with," Francis says. "And hummingbirds naturally use high frequencies that carry above compressor noise."

That doesn't really explain why finches and hummers seemed to prefer noisy sites. But if few other birds build nests near compressors, these inherently flexible species get more elbow room. Then there's the possibility that hummers and finches are responding to the relative absence of jays. Unlike species that communicate at low frequencies easily masked by throbbing compressors, house finches and black-chinned hummingbirds could take advantage of this suddenly jay-free niche.

Francis will be the first to tell you that this is all speculation. But that, of course, is what science is about -- making qualified guesses, then testing them. Now he's trying to figure out why some bird species thrive under conditions that drive many more away. "We're chasing down some of the factors involved in how noise interferes with a bird's abilities to detect signals," Francis says. "And we're looking at how a species' vocal characteristics contribute to its sensitivity to noise."

What explains the apparent advantage that noise-tolerant birds have in bringing up young? Does this pattern hold in other landscapes? Research that either confirms or refutes the preliminary findings would be equally valuable. Replicating this work elsewhere, though, could be a tall order. As the only project to factor out every variable except noise, the work in Rattlesnake Canyon is unparalleled and may remain so. Not many Rattlesnake Canyons are out there, research sites with built-in controls. As Slabbekoorn says, this study was "a unique opportunity unintentionally provided by the gas industry."

The core research, however, needs no confirmation. The downer effect that noise has on most birds? Case closed. "Noise alone changes avian communities," Francis says simply. "That's definitive."

This story was funded by a grant from the McCune Charitable Foundation.

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