Mankind affects wildlife in a number of different ways. Some are obvious, like plastic pollution and deforestation, for instance. Some are more insidious, like climate change and ocean acidification; and some, we probably haven’t even discovered yet.

Light and noise pollution are often overlooked when it comes to the impact they have on animals. We tend to dismiss them as inconsequential, although recent studies have shown that both can affect wildlife in significant ways. A new study shows just that: the presence of noisy airplanes changes some behavioral patterns of birds, making them emit louder calls.

Kurt Fristrup and colleagues from the National Park Service (NPS) analyzed almost 1 million 10-second audio recording samples from national parks across the country from 2011 to 2017. They found that when aircrafts are around, birds tend to scream a bit louder.

“We might have predicted a decrease in our ability to detect the calls due to coincident noise,” he said. “Still more unexpected, there is a statistically significant effect of the history of exposure. For aircraft sounds, the increased sound detection decays with time, but there is a measurable effect out to three hours.”

Researchers set out sound detectors and left them running in the parks. The number of bird sounds they would pick up would be generally stable, but whenever a plane passed by, the devices would pick up more sounds — because birds’ calls were a bit louder.

The effects seem to decay over time — although a noticeable effect remained up to 3 hours after a plane had passed. For instance, an aircraft that passed 2 hours ago might increase detection probability by 0.3%, but an aircraft that passed 10 minutes ago might elevate the probability by 2.3%.

These are still initial results and have not yet been peer-reviewed, but Fristrup and colleague Nathan Kleist are working on better understanding this effect, as well as the aggregate noise exposure histories across all sites.

“Given the histories of noise exposure within our data, how much are bird sound detection rates increased above the natural, noise-free condition? We’re preparing a manuscript to provide more detailed documentation of these results,” Fristrup said.

The changes are subtle but widespread. Given that airplane routes are ever-present in our modern world, it seems likely that it’s not just birds in parks that are affected — but birds all around the world.

“The extensive presence of aircraft noise exposure in time and space implies that birds are chronically producing more sound in many locations,” Fristrup said. “We’ll reanalyze existing data to determine the types of sounds that are more common — song, call notes, etc. — and which species of birds are affected.”

Future field studies are also required to assess just how these stimuli are affecting birds. They might correlate with physiological and demographic trends, and there may be environmental costs associated with the sound emitted by planes — but this is not established at the moment.