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. Sometimes I also call it "bat time" as the winged mammals like to fly circles looking for insects to gobble, too. 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 things happens with the birds in my rear garden when planes, and especially much 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 my house on the road below.

My amateur observations about birds and noise pollution are now backed up by science. A recent study found that some songbirds in the Eastern U.S. really are getting louder to deal with traffic noise, for example, while another found industrial noise is causing chronic stress and even stunted growth in some western songbirds.

Beleaguered bluebirds

male western bluebird Western bluebirds tend to gravitate toward noisy environments, but they also lay fewer eggs that hatch when they nest there, according to a 2018 study. (Photo: Maria Jeffs/Shutterstock)

The latest study, published Jan. 8 in the Proceedings of the National Academy of Sciences, 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."

ash-throated flycatcher bird The ash-throated flycatcher inhabits dry scrub, open woods and deserts in western North America. (Photo: vagabond54/Shutterstock)

Chicks had reduced body size and feather growth in the loudest areas tested, but also in the quietest areas, leaving a sweet spot of moderate noise where nestlings seem to thrive the most. 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."

Singing over traffic

Eastern wood peewee perched on a branch stretching its wings. 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)

And in another recent study, published in the journal Bioacoustics, 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 — if there was any — 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 quieter, which meant a broader bandwidth and lower sounds, as well as longer singing times, during the regular road closures.

birds on a wire overlooking a city at twilight Scientists are just beginning to understand the ecological effects of noise pollution. (Photo: Myimagine/Shutterstock)

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 for them. Since often these road closures are enacted so that there are more cycling and running areas available in parks on weekends, these car-free areas can be beneficial to both humans and wildlife alike. After all, urban humans benefit from the quiet, too.

Editor's note: This story has been updated since it was first published in April 2017.

Starre Vartan ( @ecochickie ) covers conscious consumption, health and science as she travels the world exploring new cultures and ideas.

Songbirds are struggling with noise pollution
Human noise is changing how some birds sing, while causing chronic stress and reproductive problems in others.