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When Anders Møller stumbled across a century-old paper describing sperm-milking techniques for artificially breeding budgies, he knew he was on to something good. Although sperm harvesting—which involves gently pressing on the muscles around a male bird’s cloaca, causing a contraction that triggers (nonconsensual) ejaculation—is practically an art among turkey and chicken farmers, it’s not a skill taught at ornithology school.

As with any other skill, practice makes perfect, so Møller—an evolutionary biologist and research director of the French National Center for Scientific Research—began experimenting in the field with some very unhappy males. “In the early years, half of his attempts led to something other than semen being produced,” says Timothy Mousseau, a biologist at the University of South Carolina and Møller’s longtime collaborator. “The samples were just filled with shit.”

Thousands of, er, massages later, Møller is now a pro. He recently put that skill to good use in Chernobyl, site of the world’s worst nuclear accident, in 1986. There his team has built the largest avian sperm dataset ever collected in the field. Contrary to media claims that Chernobyl’s wildlife is thriving, detailed scientific surveys show that birds and other wildlife in the area’s most contaminated tracts have suffered from mutations and population declines, and that some species have disappeared entirely. Studies have shown that men exposed to radiation from the Chernobyl catastrophe have fewer and less mobile sperm, but this is first such investigation in multiple bird species.

Over two years, the researchers used mist nets to capture 566 male birds from 46 passerine species. They sampled eight study sites in Chernobyl and the surrounding forests, with background radiation levels ranging over three orders of magnitude, from levels lower than those that occur in Central Park to ones hot enough to cause fatal cancers. Møller would coax out a few precious drops of sperm, release the birds, and immediately analyze the samples for sperm speed, mobility, and density using a microscope at the team's impromptu field station.

The sperm’s health and behavior, they , varied wildly between species. Their samples ranged from no sperm, to deformed or sluggish swimmers, to sperm that appeared perfectly normal. “Radiation clearly affected many of these birds, but in different ways,” Mousseau says.

In highly contaminated sites, nearly 18 percent of samples contained no sperm, compared with just 3 percent in radiation-free zones. According to the authors’ calculations, birds trapped in the contaminated areas were almost nine times more likely to fire blank shots than those found in control sites. At one particularly hot site, samples from 44 birds, or 40 percent of those tested, didn’t have a single sperm.

Mathieu Giraudeau, an evolutionary ecologist at the University of Zurich, says the study is an important contribution to our understanding of radiation’s effects on birds, echoing most outside experts interviewed for this story*. “The decrease of sperm quality, or even the aspermy of many birds in contaminated areas, could be a major factor explaining the decline in wild bird populations in Chernobyl,” says Giraudeau, adding that more tests, conducted under controlled lab settings, are needed to verify the link.

Pierre Deviche, a professor of environmental physiology at Arizona State University, praised the paper, although he says he would have liked to have seen the authors dig into the differences between species. Do long-lived birds have more damaged sperm, for instance, or do migratory species have lower aspermy rates? “Surprisingly, the authors do not address this important issue to any significant extent,” he says.

But other scientists are less convinced. One, Christelle Adam-Guillermin, head of the Laboratory of Radionuclide Ecotoxicology at IRSN in France, questions the study’s methodology. Adam-Guillermin points out that the researchers based their statistics on ambient radiation levels in the environment rather than on each bird’s absorbed dose, or the amount of radiation it’s exposed to both internally and externally.

That’s something the team has been investigating in hundreds of birds and mammals at Chernobyl and , the site in Japan where, in 2011, a local nuclear plant’s six reactors disastrously melted down. They attached miniature dosimeters the animals’ bodies for external readings and measured internal doses via a portal gamma spectroscopy lab. “Overall, there is a very good relationship between measured internal dose, external dose, and ambient radiation levels,” Mousseau says, adding that he plans to publish those results soon.

This latest sperm study builds on the team’s showing that wild Barn Swallows’ sperm quality significantly decreases in contaminated areas. But the scientists’ work is revealing that there’s likely no single way birds respond to nuclear disasters. , for example, they reported that several species—including Tree Pipits, Black Redstarts, and European Robins—seem to be adapting to Chernobyl’s radiation. Yet the team has also found that overall bird populations in contaminated areas are ; that Barn Swallows living there ; and that sex ratios .

Which organisms will adapt to what and when are complex questions that need to be teased out with further studies. Mousseau, Møller, and their colleagues are likely the ones who will continue tackling the challenge, building on their two decades of work—and 60 papers—addressing radiation’s impact on creatures ranging from spiders to fungus. After all, says Mousseau, the Fukushima meltdown has shown the world that additional nuclear accidents are possible. If and when others happen, today’s radiation investigations could better prepare us to deal with the fallout. 

*Three experts declined to comment on the study. Some in the evolutionary ecology field see Møller as a black sheep since a in which a Danish research committee discovered that one of Møller’s 1998 papers contained data that didn't support the conclusions. (Møller claims it was an inadvertent mistake, others disagree.)