So how does an animal whose survival depends on acoustic communication—a bird, say—cope with a relentless urban roar? Well, it could do what a person might: call out during snatches of relative quiet. Pitch its call above the din. Or get out of Dodge.
It’s May 2007, and ecologist Clint Francis, binoculars dangling on his chest, is threading his way toward an open grove of pinyon pine in the high mesas surrounding Rattlesnake Canyon in northern New Mexico. He’s here to gather evidence on which birds do what, and why, when they are up against nonstop noise. For as vast a wilderness as it seems, this Bureau of Land Management (BLM) tract is pocked with thousands of gas wells tucked among the junipers and pines. Many are decked out with industrial-grade compressors yammering away 24/7, year in and out. For Francis that’s good, because this high-altitude pygmy forest has all the attributes of a true natural experiment.
The sun tops the nearest ridge and slants through gaps between the trees, bringing sudden warmth to a chill morning. This grove marks Francis’s 12th “point-count” site in a set of 16 sites randomly generated to fall between two imaginary circles. The radius of the inner circle runs 50 meters from the center of the well pad; the outer radius extends 150 meters.
Listening intently, Francis sets his stopwatch. He has seven minutes here—seven minutes to home in on nearby birds and note who’s in the ’hood. Right off, a Gray Flycatcher calls in the distance; a Plumbeous Vireo chimes in.
Silence, though not for long. At 41 seconds a Black-headed Grosbeak erupts in song, followed at 59 by the plumbeous; a black-chinned hummer zings by. Francis scans the nearby junipers and rabbitbush, tallying who’s flying, perching, or singing and at roughly what distance. (You develop a calibrated eyeball in this business.) At 2 minutes 2 seconds, a Bushtit drops onto a nearby branch, then takes flight again. The grosbeak cuts out at 3:04, but the vireo sings on. At 5:33 a Black-throated Gray Warbler calls, then falls silent; a Mourning Dove sails in, its wings whistling softly. Six…6:20…in comes the hummer, landing briefly. At 7:00 precisely the vireo stops to catch its breath. Francis slides his notebook into his pocket, pulls out the GPS to double-check where his next-to-last stop is, and cuts through a bouldery wash toward a stand of junipers nearly 25 meters upslope, tugging off his jacket as he goes.
Relentless noise over vast stretches of planetary real estate, along with the auditory habitat fragmentation it causes, is an evolutionarily novel force acting now on a host of creatures, says Francis, a post-doc at the National Evolutionary Synthesis Center in Durham, North Carolina. Other than a lone study or two circa 1980, it wasn’t until the mid-90s that ecologists began looking at whether noise pollution and declining populations go hand in hand. The logical place to look? Along urban boulevards or busy interstates. The logical study subjects? Birds, though researchers in Europe and Australia have done decisive work on bats and frogs as well. Their evidence suggests that bone-rattling noise and species declines are linked.
The trick was controlling for a slew of confounding variables: Moving traffic. Exhaust fumes. Landscape development. Roadkill. Fences and walls that echo sounds. Unpredictably variable rates of noise intensity—not only is it measurably quieter at 6:00 A.M. than at 6:30, but a random pileup could provide a measure of calm even at the busiest hour, once the sirens cease.
Then there was the difficulty researchers faced in assessing the frequency or amplitude of calls while they are themselves awash in noise.
Francis wasn’t looking to solve this puzzle back in 2005. But he was tiring of his summer fieldwork gigs, because each winter he had to find some other job to tide him over till the next field season.
Meanwhile Bureau of Land Management wildlife biologist John Hansen needed someone to follow up on a BLM bird survey at natural gas wellheads in Rattlesnake Canyon Habitat Management Area. The survey had indicated that roughly the same numbers of bird species frequented this high-altitude pinyon- juniper forest whether or not the pads were decked out with noisy compressors.
Yet Hansen knew that seeing a bird doesn’t mean it’s nesting and rearing young. So he called Catherine Ortega, then a biology professor at Fort Lewis College in Francis’s hometown of Durango, Colorado. Ortega knew that Francis had surveyed Mexican Spotted Owls and Willow Flycatchers in Utah and that he was thinking of going to grad school. This study, Ortega thought, could make a great thesis project.
To answer Hansen’s questions—do birds actually nest near the noisiest sites, and if so, what do those avian communities look like?—Francis and Ortega devised a plan that ultimately led to the first conclusive evidence that amped-up industrial-grade noise does indeed have powerful consequences, both for how birds cope and for the richness of natural communities. Francis used the same basic protocols the original BLM team had used on the 18 well pads they surveyed. Half the pads were outfitted with painfully loud compressors; compressorless pads served as controls. His transects—randomly assigned study areas—were rectangular plots 60 meters wide extending 400 meters from each pad. And if the transects weren’t utterly uniform the way, say, Siberia or Antarctica or the Sahara might be, their habitat features varied only slightly from one to the next.
The beauty of this scenario? Instead of trying to untangle an array of iffy causative factors, Francis got a tabula rasa of sorts. He got to measure the sole urban element of concern far from sprawling cities and watch how the birds responded. It was as if he took the city to the birds.
Results? To his surprise, nest density was the same across both site types. But although 32 species nested on noise-free sites, only 21—all native to western North America—hung their hats in noisy sites. The math was simple but stark: roughly one-third of birds that normally nested in these high-mesa forests couldn’t abide being within 400 meters of compressors. Among those that occupied both types of sites, most nested at a distance from compressors—meaning that even noise-tolerant species prefer being farther from the din.
Other expectations didn’t pan out either. Francis supposed that birds nesting near compressors would raise fewer young because they wouldn’t hear predators on the make. Not so—not, at least, in the high-elevation forests of northern New Mexico.
And who would’ve guessed that 94 percent of House Finches and 92 percent of Black-chinned Hummingbirds would choose compressor sites— the closer the better?
The predators, it turned out, were mainly Western Scrub-Jays. Their noise-tolerance quotient hovered near zero, and no other predator took their place. (Similarly, cowbirds parasitized the outliers, the nests farthest from compressors; like scrub-jays, they could be intolerant of heavy-duty noise.) Which led Francis to a new speculation—that perhaps the reproductive success of some urban-adapted birds is shaped, in part, by disrupted interplay between predator and prey. Maybe those finches and hummers keyed into this relatively predator- and competitor-free space when they chose nesting sites.
Plus they seemed to have innate advantages (they’re small and have higher-pitched songs) that made them oddly adaptable, able to fill a niche that drove other birds bonkers.
Compressor noise has energy at frequencies that span most of our hearing range. But its low-pitched clamor carries farther than its higher frequencies, which drop out at shorter distances— much like urban and traffic noise. Research by Emily Mockford (Britain) and Hans Slabbekoorn (Netherlands)—and now by Eira Bermúdez-Cuamatzin (Mexico) and Dominique Potvin (Australia)—has suggested that some species get by in sprawling metropolises by pitching their songs higher than their rural brethren or by having naturally high-pitched songs and calls to begin with—by being the Sarah Brightmans, the coloratura sopranos of the bird world. Perhaps the finches and hummers of Rattlesnake Canyon were doing the same.
His first study was nearly complete and beginning to turn heads (Francis’s research earned three student awards while he was still at work on his Ph.D. thesis), so he wanted to see what else this unexpected opportunity, courtesy of the gas industry, had to offer. If relentless noise compromises natural communities and yet provides indirect benefits to some species, what can we learn about how birds do or don’t cope? Because Francis got to track so many species, he’d turned up intriguing data he would have missed had he drilled in on just a few. This in turn led him to focus on two related species with a curious story to tell—the Ash-throated and Gray flycatchers.
Some background first. Flycatchers belong to a suborder of songbirds called “suboscines.” (“Songbirds” is a misnomer, actually; more properly these birds are called “passerines”; most are technically “oscines.”) Ornithologists believe that suboscines are born knowing just what song to sing to defend a territory or find a mate, unlike most young male songbirds, which learn songs from their fathers or neighborhood males.
So if flycatcher songs are genetically ingrained, wouldn’t they have less inherent flexibility where noise pollution is an issue? Be less likely to adjust their songs—sing higher, call louder— depending on the noise levels they need to cope with?
Wouldn’t they get out of Dodge?
If you forced Francis to guess, no a priori knowledge allowed, if either flycatcher could nest within the noisy transects, let alone modulate its song, and if so, which one, well, first he’d say that by rights, both birds should avoid all but outlying areas. If you pressed him for which might be less likely to tolerate noisy sites, he just might put his money on the Ash-throated. After all, the Ash-throated Flycatcher is larger; larger birds pitch their songs lower; lower songs are more easily masked by the rumble of compressors, highways, and the like—and other researchers had noted that birds with higher ranges do better in noisy city settings.
But Francis had a posteriori material to work from. His nest surveys showed that Ash-throated males paid scant attention to noise when seeking a place to hang their hat, while Grays on noisy sites nested at the far ends of transects. The 2007 pointcount surveys confirmed what Francis saw in the nesting study: Gray Flycatchers kept their distance.
Still, confirmation and explanation are two separate things. Why were the Ash-throated Flycatchers unfazed by industrialgrade sound pollution? What allowed them to compensate despite the tyranny of genetic programming that, in theory at least, made adapting to circumstance harder?
Francis had to go back again.
Which is why at dawn on a chilly May morning in 2009, Francis and his crewmates, Ryan Kennedy and Nate Kleist, shoehorn themselves into Francis’s truck. They wind south on Colorado 170 out of Durango, then clickety-clack across the cattle guard where 511 takes over at the New Mexico line.
Even with his seat all the way back, Francis’s knees knock against the steering wheel. Good thing he’s not any taller. He veers off onto a rutted, unmarked track that winds over a ridge toward a far valley. Though he’s easy on the gas, everyone is jouncing around. In the distance they can see five well pads dotting the mesa. They’re swapping yarns to pass the time. About that time they got to the base of a hill like this one and saw a gascompany field man, trailering a backhoe, stuck at the bottom. A couple of hours they waited in the hot sun while the guy cussed and kicked the tires and threw his hat on the ground and, finally, got the damn thing to the top. About when sudden cloudbursts turned the fine clay dust to boot-sticking gumbo and everyone was several inches taller by the time they clumped back to the truck. About that day Pete set his camo bag down in a thicket of gambrel oak, stepped out to get a bead on a bird—and the crew spent the afternoon looking for that pack. Had to. The keys were in it.
Once on-site, the three unwind from the cab, stretching, then open the cap on the pickup’s bed and drop the tailgate. Out come the shotgun mics and fancy-pants digital recorders, the decibel meters, and GPS units. Each crewmember packs a set.
This year they’re making field recordings of bird songs and calls—a first for Francis. They’re hitting 37 well pads, nearly twice as many as in previous years. Why more pads? Partly it has to do with not double-sampling birds; they’re recording just one bird per species per site. But partly it’s because this year, the compressors stay on. This year, finding birds won’t be as easy. And once they’re found, crewmembers need to work to within five to fifteen meters, wait patiently for the birds to begin singing (if they do), and then record the entire song or call bout—that is, for as long as the bird sings from one perch.
The three spread apart as far as possible; they don’t want to risk sampling the same birds. Once back at the truck, they’ll check their findings against each other’s notes. If there’s any question about double-sampled birds, Francis listens to both recordings, then discards the lower-quality one.
While Gray and Ash-throated flycatchers aren’t the crew’s only quarry, they happen to be the only suboscines in plentiful supply. Meaning Francis can’t help but pay them extra heed. And the reward…well, if he’s lucky Francis will clear up the mystery of the flycatchers, why the bigger, lower-pitched Ash-throated Flycatcher bucks the trend by its seeming indifference to compressor noise, while its smaller, higher-pitched cousin can’t compete with the racket.
Each afternoon as he downloads the recordings onto his computer, Francis randomly picks five songs or calls from each bird. He measures the three main characteristics of each: frequency (both minimum and maximum); peak frequency (the frequency carrying the most energy); and vocalization bandwidth (maximum frequency minus minimum frequency). While his software automatically measures peak frequencies, he has to locate the minimum and maximum frequencies himself.
Luckily the spectrograms on his computer screen—resembling an art piece in chart form—make it easy for Francis to place his cursor at the margins of notes, despite all that background noise. By the time he’s put each recording in the context of his decibel readings, by the time he’s run the log transformations and analyzed the linear regressions and adjusted the significance thresholds— by then his data are looking good, very good. Because this time his expectations are dead-on. Somebody is modulating its song. And it isn’t the higher-pitched Gray.
The Grays uphold family tradition, sticking strictly with the tried and true. The Ash-throateds, however, seem willing to consider their options. When near compressors, they pitch their lowest notes a tad higher—200 hertz—than their kin at a greater distance.
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Granted, 200 Hz is a difference too subtle for you and me to hear. We can’t hear even a 1,000 Hz jump. But we aren’t flycatchers or any of the other species researched in Britain, Belgium, Mexico, and Australia that have been shown to modulate their song where sound pollution is an issue. The adjustments they make range from 100 to 500 Hz—not a lot.
Then again, maybe the pitch shift isn’t critical here. The Ash-throated Flycatchers could merely be singing louder, the higher frequency being a secondary effect. Meaning they’re sort of like us—the louder we talk, the higher-pitched our voices tend to be. Francis can’t measure the singing-louder hypothesis, though, unless he places microphones directly beneath the birds. Good as this natural experiment is, it’s not that good.
Meanwhile the smaller Gray Flycatcher’s song, while naturally higher, is also softer. Quieter. Perhaps the Gray is already maxed out and couldn’t sing louder, even if it tried.
We’re back at base in Durango. Outside, trucks are grinding up the four lanes of Goeglein Gulch Road. “Here,” Francis says, his laptop open. “Take a look.” Google Earth is up. Francis skims his fingers across the trackpad and the high mesas surrounding Rattlesnake Canyon in northern New Mexico zoom into view. “That’s our study area.”
We were out there just a couple of hours ago, filing past the pinyon pines, threading through the sage. But an eagle would see it like this, if far better: an ancient landscape studded with thousands of natural-gas well pads and crisscrossed with roads leading from one to the next. A landscape with a serious case of the pox.
Zooming back out, we see the ropelike web of roads that even here in the fabled West coil through the valleys or cut through every range. Though the nation’s population grew by a third during the past 40 years, road traffic nearly tripled. In the contiguous 48 states, 83 percent of all land now lies within two-thirds of a mile of a road. Air traffic has tripled since the early 1980s; it’s hard to find places free of high-altitude noise. Even rural and wild landscapes are prone to a new kind of industrialization—scattered, perhaps, and accessible only by crude, rutted dozer tracks—but industry even so.
All of this is acoustic habitat fragmentation. Try googling “acoustic habitat fragmentation” (in quotes). You’ll get two hits, both for whales—a fair subject, granted, in their own right. Then Google plain “habitat fragmentation”: you’ll score 249,000 hits. Even accounting for so crude an approximation, don’t the numbers suggest fertile research ground for curious ecologists?