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.