Scientists have discovered that birds may be suffering from stress similar to post-traumatic stress disorder due to oil and gas noise pollution, and it is affecting their reproduction. The study, published by researchers at the University of Colorado in the Proceeding of the National Academy of Sciences, found that birds that were impacted by the noise pollution tended to lay fewer eggs that hatch.

For the study, scientists examined western and mountain bluebirds, as well as ash-throated flycatchers during three breeding seasons. The researchers built 240 nest boxes on 12 pairs of sites for the birds, which are often found near oil and gas operations in New Mexico.

They found that the noise had a significant amount of impact on not only the birds’ stress hormones, but also their fitness. And the study also challenged scientists’ assumptions about birds being able to handle the noise, as the findings indicated that they weren’t as hardy as researchers thought they were.

“In what we consider to be the most integrated study of the effects of noise pollution on birds to date, we found that it can significantly impact both their stress hormones and their fitness,” said lead author Nathan Kleist, who conducted the research while at CU Boulder and graduated with a PhD in evolutionary biology in May. “Surprisingly, we also found that the species we assumed to be most tolerant to noise had the most negative effects.”

“You might assume this means they are not stressed. But what we are learning from both human and rodent research is that with inescapable stressors, including posttraumatic stress disorder (PTSD) in humans, stress hormones are often chronically low,” said co-author Christopher Lowry, a stress physiologist in the department of integrative physiology at CU Boulder.