I often go trail running in the wooded hills behind my house during the "blue hour" — that time of night after the sun sets, but before it's truly night. I also sometimes call it "bat time" as the winged mammals like to fly circles looking for insects to gobble. At one curve in the trail, I almost always hear the specific call of a pair of great horned owls — that classic, melancholy "hoot, hooooooot" sound.
But I've noticed that when a plane flies overhead — a semi-distant drone (they are taking off about 25 miles away), the owls hoot louder. The same thing happens with the birds in my back garden when planes and louder helicopters fly above. Those times when I'm working outside, there for a few hours in relative silence, save the clacking of my laptop keys, I've noticed the birds raise their songs even when a loud truck goes past on the road below.
It turns out my amateur observations about birds and noise pollution are backed up by science, as this series of studies prove.
Noise impacts clear communication
The newest study finds that noise pollution makes it hard for birds to communicate with each other. Man-made sounds mask signals between birds, researchers from Queen’s University Belfast discovered.
Their study, published in the journal Biology Letters, found that background noise can hide critical information that birds use and share, a problem that could eventually lead to a severe decline in population numbers.
Birds sing to defend their territory and to attract a mate, but this becomes more difficult as noise pollution hides their sounds and the critical information they're trying to convey.
"We found that bird song structure can communicate aggressive intent, enabling birds to assess their opponent, but human-made noise can disrupt this crucial information passed between them by masking the complexity of their songs used for acquiring resources, such as territory and space for nesting," said coauthor Dr. Gareth Arnott, senior lecturer and researcher from the university's Institute for Global Food Security. "As a result, the birds receive incomplete information on their opponent's intent and do not appropriately adjust their response."
Bluebird chemistry upset by oil operations
A study published in the Proceedings of the National Academy of Sciences in 2018 looked at how constant noise from oil and gas operations affects songbirds living nearby. It focused on three species of cavity-nesting birds — western bluebirds, mountain bluebirds and ash-throated flycatchers — that breed near industrial oil and gas sites on federal land in New Mexico.
Across all species and life stages, birds nesting in areas with more noise showed lower baseline levels of a key stress hormone called corticosterone. "You might assume this means they are not stressed," explains study co-author Christopher Lowry, a stress physiologist at the University of Colorado Boulder, in a statement. "But what we are learning from both human and rodent research is that, with inescapable stressors, including post-traumatic stress disorder (PTSD) in humans, stress hormones are often chronically low."
When the fight-or-flight response is overworked, the body sometimes adapts to conserve energy and can become sensitized. This "hypocorticism" has been linked to inflammation and reduced weight gain in rodents, the researchers note. "Whether stress hormone levels are high or low, any kind of dysregulation can be bad for a species," says senior author Clinton Francis, an assistant professor of biological sciences at California Polytechnic State University. "In this study, we were able to demonstrate that dysregulation due to noise has reproductive consequences."
Chicks had reduced body size and feather growth in the loudest areas tested, but the same was true for the quietest areas, leaving a sweet spot of moderate noise where nestlings seem to thrive. The researchers think this might be because adults in the quietest places are exposed to more predators, leaving less time to forage because they're more cautious leaving the nest. In the loudest places, machinery noise drowns out calls from other birds — including potentially life-saving messages about predators — which could chronically stress both moms and nestlings.
Previous research has shown that some bird species decide to flee noise pollution, but the researchers say this study helps reveal what happens to those that stay behind. And according to lead author Nathan Kleist, it also helps illustrate how ecologically disruptive loud noises can be.
"There is starting to be more evidence that noise pollution should be included, in addition to all the other drivers of habitat degradation, when crafting plans to protect areas for wildlife," he says. "Our study adds weight to that argument."
Traffic makes this songbird sing louder
Birds like the Eastern wood peewee, pictured here, can have trouble communicating their mating calls over human-made sounds like traffic, leaf-blowers and planes. (Photo: Paul Reeves Photography/Shutterstock)
In a study published in the journal Bioacoustics in 2016, Katherine Gentry of Virginia's George Mason University studied the Eastern wood pewee, a common songbird in the Washington, D.C., area.
Gentry and her team recorded at three different parkland sites: Some of them were near constant traffic, and others were near roads that were closed on a regular schedule for 36-hour periods. The researchers took specific note of the birds' calls, including data on the duration of songs, and maximum and minimum loudness. They also collected the noise of the traffic nearby at the same time. (Some of the areas they recorded in had regular 36-hour road closures.)
When compiled and analyzed, the study found that birds did indeed get louder when traffic was zooming by, and they got quieter during the regular road closures, which meant a broader bandwidth and lower sounds, as well as longer singing times.
This is important, since quite a bit of birdsong is about attracting or communicating with a mate. When birds get louder, their song is less nuanced and shorter, and may not quite communicate what they're trying to get across. That's why, as the scientists wrote in the research paper, "... traffic noise is associated with a decline in reproductive success and species richness, contributing to the decreased biodiversity of ecological communities and reduced fitness of individuals near roads."
Ultimately, this is both a recognition of our less-obvious impacts on wildlife and more specifically, a scientifically backed reasoning behind closing roads — even just short-term traffic calming has measurable impacts. This kind of conservation strategy could help songbirds like the Eastern wood peewee, whose population has declined by more than 50 percent since cars have become prevalent in places like D.C.
Birds can adapt to some of the environmental pollutants humans throw at them — including noise — but small changes like cutting traffic in certain areas at certain times can make a big difference. These road closures are enacted to create more cycling and running areas available in parks on weekends, so these car-free areas can be beneficial to both humans and wildlife.
After all, urban humans benefit from the quiet, too.
Editor's note: This story has been updated with new information since it was first published in April 2017.