When we hear the word pollution, we tend to think of chemicals in the ocean, or toxic gases in the air. However, noise pollution can be just as damaging to our health, and researchers have recently discovered that exposure to persistent noise causes chronic stress in birds. So badly, in fact, that their symptoms rival that of post-traumatic stress disorder (PTSD) in humans.

While we can drown out persistent noise with earplugs and noise-canceling headphones, the animals that share our planet aren’t so lucky. Many species, from enormous whales to tiny tree frogs, are negatively impacted by man-made sounds from boats, cars, and construction, which can cause deafness, severe stress, and changes in natural behavior.

Now it seems that three little birds – the western bluebird, the mountain bluebird, and the ash-throated flycatcher – aren’t safe either, as the continuous noise from nearby natural gas compressors appears to make them incredibly stressed. This in turn affects their young, as tense adult birds spend more time guarding their nests, and less time feeding their chicks.

Publishing their findings in the Proceedings of the National Academy of Sciences, a team of scientists investigated the relationship between noise, stress hormones, and fitness in the three species. They set up 240 nest boxes at various distances from noisy gas compressors, allowing them to assess just how badly the noise was affecting the birds.

They measured the levels of a stress hormone, known as corticosterone, produced by the birds. Rather surprisingly, they found that baseline levels of this hormone were lower the closer the birds were to the noise. However, comparing these results to previous studies of chronic stress, the researchers found that more alarming than surprising. Low base levels of corticosterone can actually occur as a reaction to intense stress, as the body tones down levels of the hormone to protect itself. This is seen in cases of PTSD.

What’s more, when chicks were exposed to a sudden threat, their corticosterone levels skyrocketed and took time to return to normal. The combination of low baseline levels of stress hormone and abnormal spikes due to threats correlates with chronic stress research on both rodents and humans.

Chicks nearer to noise were also found to be smaller, with poorer feather development – two factors that affect survival. This is likely due to parent birds spending more time being vigilant and less time looking for food due to a decreased “listening area”, as they couldn’t hear predators coming so easily.

"This study shows that noise pollution reduces animal habitat and directly influences their fitness and ultimately their numbers," said study co-author Rob Guralnick in a statement. "By doing so, it makes it harder for animals to survive. Taken together, that's a pretty damning picture of what human-made noise can do to natural populations of animals."