The first time Tim Mousseau went to count birds in Fukushima, Japan, radiation levels in the regions he visited were as high as 1,000 times the normal background. It was July 2011, four months after the Tohoku earthquake and subsequent partial meltdown at the Fukushima-Daiichi nuclear power plant, and the nation was still recovering from massive infrastructure damage. Still, when Mousseau and his research partner rented a car and drove up from Tokyo, they encountered little resistance on the road.

“I knew we had to get there and capture as best we could the early effects [of radioactive contamination] that nobody had really looked for,” he remembers thinking after seeing news of the Fukushima disaster. “Ultimately we realized that our best possible approach for that first year was simply to start doing bird counts.”

Now, after four years surveying bird populations in 400 sites around Fukushima-Daiichi, Mousseau and his team have assembled a grim portrait of the disaster’s impact on local wildlife, using bird populations as a model system. Even though radioactivity has dropped throughout the region, their data show that bird species and abundances are in sharp decline, and the situation is getting worse every year.

“At first only a few species showed significant signs of the radiation’s effects,” Mousseau says. “Now if you go down and around the bend maybe five or ten kilometers [from a safe zone] to where it’s much, much hotter, it’s dead silent. You’ll see one or two birds if you’re lucky.”

Mousseau’s team conducted almost 2,400 bird counts in total and gathered data on 57 species, each of which showed specific sensitivity to background radiation. Thirty of the species showed population declines during the study period, the team report in the March issue of the Journal of Ornithology. Among these, resident birds such as the carrion crow and the Eurasian tree sparrow demonstrated higher susceptibility than migratory species, which didn’t arrive in the region until a few weeks after the partial meltdown in early March.

Nuclear accidents are rare in human history, so we have very little data about such radiation’s direct effects on wildlife. Mousseau has spent the past 15 years drawing comparisons between nuclear events to help build up our knowledge base and fill in the gaps. For instance, while there are no official published records of the Chernobyl disaster’s early impact on wildlife, plenty of work has been done in recent years to assess Chernobyl's ecosystem post-accident, from local birds to forest fungi.

When Mousseau returned to Fukushima in 2012, he began capturing birds in irradiated zones that had patches of bleach-white feathers. It was a familiar sign: “The first time I went to Chernobyl in 2000 to collect birds, 20 percent of the birds [we captured] at one particularly contaminated farm had little patches of white feathers here and there—some large, some small, sometimes in a pattern and other times just irregular.”

His team thinks these white patches are the result of radiation-induced oxidative stress, which depletes birds’ reserves of the antioxidants that control coloration in their feathers and other body parts. In Chernobyl, the patches have a high coincidence with other known symptoms of radiation exposure, including cataracts, tumors, asymmetries, developmental abnormalities, reduced fertility and smaller brain size.

By 2013, the birds Mousseau was counting in Fukushima had white patches big enough to be seen through binoculars.

Presented together, Mousseau thinks such data sets on Chernobyl and Fukushima could offer significant evidence for radiation’s prolonged, cumulative effects on wildlife at different stages after a nuclear disaster. But other experts have a completely different take on the available information.

“I’m not convinced about the oxidative stress hypothesis, full stop,” says Jim Smith, editor and lead author of Chernobyl: Catastrophe and Consequences and an expert on pollution in terrestrial and aquatic ecosystems. “The radiation levels in both Fukushima and Chernobyl are currently low-dose, and the antioxidant capacity of a cell is way, way bigger than the oxidizing capacity of the radiation at those levels,” he says. This would mean the white feather patches—and perhaps the overall bird declines—are being caused by something other than radiation.

Birds’ feathers often change color as a byproduct of aging, much like our hair color changes as we get older. They also get replaced in molt cycles a few times a year and require new doses of melanin every time to retain their pigment. According to Yale evolutionary ornithologist Richard Prum, this opens the door for pigment mutations to occur quite regularly—whether or not a bird lives in or passes through a radiation zone.

“It’s a bit like fixing a car: the problem may be obvious, but there are lots of moving parts,” says Prum, who studies the evolution of avian plumage coloration. “Melanin stress can manifest in the same way—such as white feathers—under a variety of circumstances, and the causes behind it can be very diverse. Just this winter I saw four species with abnormal white pigmentation visit my feeder at home, but I’m not too worried about radiation levels in New Haven.”

Prum says he had heard the ecosystem at Chernobyl was doing quite well, an opinion defended by Mousseau’s critics. Back at the University of Portsmouth in the U.K., Smith primarily studies aquatic invertebrates, and in some of Chernobyl’s most contaminated lakes he has actually observed increased levels of biodiversity following the accident.

“Many of the literature studies on animals find it difficult to distinguish between the early effects of high doses shortly after the accident and later effects of much lower subsequent doses,” Smith says. “Plus some of them don’t properly account for the ecosystem impacts of removal of humans.”

Back in 2000, Robert Baker and Ron Chesser of Texas Tech University published a paper characterizing Chernobyl as a wildlife preserve, established thanks to the absence of humans since the accident. Both scientists have maintained that biodiversity and species abundance in Chernobyl and Fukushima are, in the long term, not adversely affected by radiation.

“Despite our best efforts, post-accident field studies aren’t sufficient to give us a clear picture,” says Chesser. “They offer no good controls, because we aren’t working with data from before the accident.” Chesser suggests that physiological aberrations of the sort Mousseau has observed are not conclusive results of chronic radiation exposure. Instead, they reflect other sources of oxidative stress including reproduction, immune response to infection and disease and strenuous physical activity such as migration.

“All the evidence that I grew up with and read in the last 60 years tells me [Mousseau’s findings] are probably wrong,” Chesser says, explaining why he disputes radiation as the cause behind the bird declines in Japan. “I don’t intend to cast aspersion on anyone, but if your evidence is really outside the norm, you better have some extraordinary data to back that up.”

Mousseau acknowledges that his research methods deviate from those of “old-school radiation biologists,” whose work has typically measured responses to radiation based on Geiger counter readings of individual animals. Not caring about the exact levels of radioactivity, as Mousseau says he does not, understandably ruffles some feathers.

“We’re strictly motivated by measurements of ecological and evolutionary response,” Mousseau says. “Our extraordinary evidence relates to these censuses, these massively replicated bionic inventories across a landscape scale and in both locations, and that has not been done in any rigorous way by any of these other groups.

“The data are not anecdotal, they’re real and rigorous,” he adds. “They’re replicated in space and time. How you interpret them is up for grabs, and certainly a lot more experimentation needs to be done in order to better appreciate the mechanism associated with these declines.” For their part, Mousseau’s team hopes next to understand why different bird species in their data appear to demonstrate varying levels of radioactive sensitivity. They’re headed to Chernobyl again next week, and back to Fukushima in July.

Update 5/1: James Smith's affiliation has been corrected; he is a professor at the University of Portsmouth.