A starling. (Daniel Baleckaitis/University of California at San Diego/AP)

It’s been well-documented that a decreased sex drive can be one of the side effects of antidepressants like Prozac. But the amount of these drugs that end up in sewage plants may also have an impact on the mating habits of wild birds, a new study from the University of York shows.

Researchers found that female starlings that had been exposed to small doses of fluoxetine, the generic name for Prozac, became less attractive to male starlings, which sung to them less often and treated them more aggressively. Kathryn Arnold, one of the study’s authors and a senior lecturer in ecology at the University of York, described it as “the first evidence that low concentrations of an antidepressant can disrupt the courtship of songbirds.”

That’s problematic because birds that are slow to find a mate may not get the chance to breed, she wrote.

“We’re definitely not saying that it’s bad to take antidepressants, but certainly there is a greater need for new technologies to clean out sewage,” Arnold told The Washington Post.

Birds like to graze at sewage treatment plants, which are teeming with worms, flies and maggots, she explained. But because antidepressants often make their way through the human body and into sewage plants without fully breaking down, those insects are frequently laced with prescription drugs.

To figure out the effect that these pharmaceuticals have on birds, the researchers first had to figure out how much fluoxetine is typically found in worms and bugs at sewage treatment plants. They then replicated that, dosing worms with a low concentration of Prozac — less than 10 percent of what a human would be given for therapeutic purposes, Arnold said.

The Prozac-treated worms were fed to starlings living in outdoor aviaries that mimicked the natural environment as much as possible while keeping the birds in captivity. The following spring, during breeding season, male starlings were “set up on essentially blind dates,” Arnold said.

The results were striking. When paired with female starlings that had been eating the Prozac-laced worms, males only sang half as often and for half as long as they did when they were paired with a female that hadn’t been exposed to the drug.

That’s significant, Arnold said, because singing is an important part of birds’ courtship rituals, and is used to judge potential mates. “The equivalent of reduced libido, in a wild animal, is how they behave during courtship,” she said.

Male starlings were also much more aggressive toward female starlings that had been exposed to low doses of Prozac, the researchers found.

“They chased them, they pecked them, they tugged on their feathers,” Arnold said.

Interestingly, female starlings didn’t seem to be turned off by male starlings that had consumed small amounts of Prozac. Arnold credits that to the fact that the male starlings’ behavior didn’t change drastically, while female starlings seemed less interested in the opposite sex and were often inconsistent in their behavior.

“They were blowing hot and cold,” she said. “They could be aggressive, and they could be passive.”

Could this result in fewer baby birds being born and, ultimately, a decline in the number of starlings? Arnold said it’s possible, but further research is needed to determine if the same behavior that her team witnessed is also happening in the wild. Complicating the question is the fact that birds aren’t just being exposed to Prozac — they’re also potentially ingesting a whole slew of other drugs that have ended up in the sewage system.

“There is growing evidence around the world that shows that we need to be taking a look at what it is we’re pumping out,” she said. “These cocktails of pharmaceuticals are drugs that your medical provider would never prescribe in combination with one another.”