The bluebird didn't realize what she was getting herself into when she chose her new home, about 75 yards from a natural gas compressor. It was only as the days and weeks wore on that the low whine of machinery started to take a toll. It was harder to hear the sounds of approaching predators, or even the normal noises of the surrounding world, so she had to maintain constant vigilance. Her stress hormone levels became skewed; her health deteriorated. She couldn't resettle elsewhere, because she had a nest full of hatchlings to tend. Yet her chicks suffered too, growing up small and scantily feathered — if they survived at all.
Scientists couldn't ask the bluebird what she was feeling. But when they sampled the bird's blood, as part of a study of 240 nesting sites surrounding natural gas treatment facilities in northern New Mexico, they found she showed the same physiological symptoms as a human suffering from post-traumatic stress disorder.
“Noise is causing birds to be in a situation where they're chronically stressed . . . and that has really huge health consequences for birds and their offspring,” said Rob Guralnick, associate curator of biodiversity informatics at the Florida Museum of Natural History.
It would be a stretch to say noise hurts birds' mental health — the animals have not been evaluated by an avian psychologist. But in a paper published Monday in the Proceedings of the National Academy of Sciences, Guralnick and his colleagues say there is a clear connection between noise pollution, abnormal levels of stress hormones, and lower survival rates. This is the first time that link has been established in a population of wild animals, they argue, and it should make us all think hard about what our ruckus is doing to the Earth.
“Habitat degradation is always conceived of as clear cutting, or, you know, changing the environment in a physical way. But this is an acoustic degradation of the environment,” Guralnick said. “We think it is a real conservation concern.”
The research was conducted at the Bureau of Land Management's Rattlesnake Canyon Habitat Management Area, a sun-drenched expanse of sage-brush-covered mesas and steep canyons forested with juniper and pinyon pine. The site is uninhabited, but it's dotted with natural gas wells and compression stations that emit a constant, low-frequency hum in roughly the same range as many birds' songs. That makes it the perfect place to study the effects of human-produced noise, far from humans themselves.
Clint Francis, an ecologist at California Polytechnic State University, has been studying this ecosystem for more than a decade. In previous studies, he has reported that noise can restructure entire communities — prompting birds to alter the pitch of their songs, shifting demographics toward more noise-tolerant species, even changing the distribution of plants based on what birds remain. He also found that some species fared better when they nested near noise sources, because the clamor drove away predators that would otherwise eat vulnerable eggs and chicks.
“But we were still concerned there might be hidden costs when you measure other aspects of reproductive success,” Francis said. “That's why we wanted to look at stress hormones.”
Blood tests revealed that levels of corticosterone in birds closest to the gas compressors were far, far lower than normal. This initially came as a surprise to the researchers, because corticosterone is the bird equivalent of cortisol — the hormone that prompts your body to release a flood of adrenaline, bump up your blood pressure, and jolt your brain with sugar.
But then they took their results to Christopher Lowry, a stress physiologist at the University of Colorado at Boulder. To him, the results weren't surprising at all — it's what you would expect in a creature exposed to prolonged, persistent strain.
Humans suffering from PTSD or chronic fatigue syndrome, and lab mice that have been put through traumatic experiences, respond by muting their hypothalamic-pituitary-adrenal (HPA) axis — the cascade of chemical responses that is triggered by stress.
“You can imagine being in a state of constant arousal and hypervigiliance,” Lowry explained. “If there was not some way to desensitize these systems, that would result in a state of chronic fatigue. No organism is capable of essentially running on turbo all the time. So after a period of time the physiology adapts — perhaps to conserve resources.”
It's an adaptation to an untenable situation, Lowry said, but not a particularly good one; this tamping down of the HPA axis is tied to an overall deterioration of health. A human is likely to experience cardiovascular problems, gastrointestinal issues, extreme fatigue. Meanwhile, the western bluebird — a common, hardy species that is generally considered to be noise-tolerant, was smaller, its feathers bedraggled.
“The body is just starting to break down,” Lowry said.
To Lowry, the fact that humans respond to stress in the same manner as animals as distantly related as birds suggests that this response is ancient and deeply ingrained. And it raises questions about how humans handle exposure to unrelenting noise. The mother bluebird that nested near a compressor and was unable to leave when the sound became unbearable may not be so different from a low-income human family forced to rent an apartment near a flight path or loud industrial site.
“There's evidence that being able to have a full auditory experience is essential for optimal health in both species,” Lowry said.
Why is hearing so important to health? Francis believes it has to do with our ability to sense the dangers around us.
“Animals are constantly surveying their environment and making decisions based on risk. And one thing noise does is it degrades this really important sensory channel,” he said. “It very well may be that noise exposure really cuts an animal's ability to use sound or rely on passively listening for dangers, and that sets up conditions where risk assessment is really uncertain and leads to psychological conditions like anxiety.”
It will be hard to figure out exactly whether this is happening, Francis said, because there are so many interconnected factors at work in this setting. For one, the birds might be stressed by noise, but so are the animals that could eat them; the data suggests there is a Goldilocks distance from compressors where birds are better off because the sound scares of predators.
“It gets really tricky,” Francis said. “We're going to need to come up with some really creative experiments to sort this out.”