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It’s been four years since the nuclear meltdown in Fukushima, Japan, and life is still nebulous. The 12-mile zone around the plant, for example, has been all but completely abandoned. Local birds are not faring too well either. In the areas most contaminated by radioactive fallout, researchers have documented sharp declines in both the total number of birds and species richness. This wasn’t unexpected, given the noxious nature of radiation. But in a surprising twist, it seems that year-round birds are suffering more than migrants—which is the opposite of what’s happening at Chernobyl, the site of an even bigger nuclear accident in the Ukraine and Belarus in 1986.

Tim Mousseau, a professor of biological sciences at the University of South Carolina, has published on the birds of Fukushima. His research shows that the birds that were there in March 2011 “got the full brunt of the exposure.” Since migrants arrived in the aftermath of the disaster, the impact on them was limited.

But there are longstanding effects at play too. As seen in Chernobyl, chronic radiation exposure can cause genetic mutations to accumulate. This appears to present a particular problem for migratory birds, which, having just flown long distances, arrive with depleted stores of antioxidants. The lack of enzymes and vitamins impairs the birds’ abilities to repair their own DNA. “Migration is a very, very stressful time of the year for these organisms,” Mousseau says, “so the addition of stress related to radiation exposure makes them very vulnerable.” He points out that nobody monitored Chernobyl’s wildlife in the years immediately following the disaster, so it’s possible that early on, resident birds did worse than migrants there as well.

Mousseau and his team have been doing comprehensive bird surveys of the Chernobyl site since 2005, and of the Fukushima site since just a few months after the catastrophe unfolded. Each July, they counted birds at 400 wooded locations in Fukushima province: The sites ranged from relatively uncontaminated to highly contaminated.

From that data set they found that while a few birds increased in abundance, most others declined (or disappeared entirely), with the Barn Swallow, Eurasian Tree Sparrow, and Carrion Crow faring the worst. To make matters graver, the trends show that the impact of the Fukushima nuclear disaster on birds has only intensified over time.

Barn Swallows are known to be particularly sensitive to radiation. But identifying the exact mechanism behind their decline in Fukushima has proved difficult. “We know there were hundreds and hundreds of Barn Swallows living on the coast before the disaster,” Mousseau says. “Now the birds have dropped off so much in the areas of highest contamination that there are few left to sample.”

Conducting science in the thick of radiation can be nerve-racking. But don’t expect to see Mousseau in a hazmat suit or other heavy protective gear. Instead, he tries to “get in and out quickly” and to “avoid ingestion of dirt, water, and dust.” “The animals,” he says, “don’t have it nearly as lucky.”