The destructive earthquake and tsunami that triggered a catastrophe at Japan's Fukushima-Daiichi nuclear power plant are now four years behind us, but the effects of that disaster are still being felt today. Now a new study has revealed that even as ecosystems slowly recover, Fukushima's native bird population is actually dwindling more than ever - and researchers think they know why.

Compared to the infamous Chernobyl power plant meltdown of 1986, the amount of radiation released during and after the Fukushima-Daiichi incident was but a drop in the bucket. Back in November, an independent survey of West Coast waters near the United States revealed that the telltale radioactive compound censium was at concentrations more than 1,000 times lower than acceptable drinking water limits set by the US Environmental Protection Agency (EPA).

That revelation should put panicking North Americans, who are at least 5,350 miles away (across an entire ocean) from the immediate disaster site, at ease. However, that certainly doesn't mean that the disaster was largely harmless.

"A growing body of empirical results from studies of birds, monkeys, butterflies, and other insects suggests that some species have been significantly impacted by the radioactive releases related to the Fukushima disaster," Timothy Mousseau, a lead researcher behind one of many ongoing investigations, announced last August.

He explained that a common theme researchers have noticed is that prolonged exposure to low doses of radiation has a very different effect on organisms compared to sudden high-level exposure, as seen at Chernobyl. This came to be called Fukushima's "insidious effect" and was expected to abate, with lingering radiation levels, by 2016.

Unfortunately, new research detailed in the Journal of Ornithology shows that birds may be facing trouble for even longer.

"The declines have been really dramatic," study author and biologist Tim Mousseau, of the University of South Carolina, said in a recent statement.

Mousseau and his colleagues have been launching regular bird censuses for 57 local species in contaminated areas, since a few months after the 2011 disaster. And although the data may be skewed (as the researchers were not allowed to conduct their work in the "hottest" radioactive spots for the first two years), they were still able to showcase large avian population losses in a dose-dependent manner. The barn swallow (Hirundo rustica), for example, was one species that was hit the hardest. (Scroll to read on...)

"There are so few barn swallows left," Mousseau added. "We know that there were hundreds in a given area before the disaster, and just a couple of years later we're only able to find a few dozen left."

A second study published in the Journal of Ornithology goes on to detail how Mousseau and Anders Moller, of The French National Centre for Scientific Research (CNRS), worked to compare and contrast the impacts that the Fukushima and Chernobyl disasters had on birds.

They quickly found that while the greater of these two disasters outright killed many avian species, those who survived or later moved back into the region have proved resilient. Meanwhile, although most birds survived Fukushima, their populations continue to rapidly decline even years after the disaster.

"It suggests to us that what we're seeing in Fukushima right now is primarily through the direct result of exposure to radiation that's generating a toxic effect - because the residents are getting a bigger dose by being there longer, they're more affected," Mousseau said.

And as that toxic effect is likely shortening lifespans and adversely affecting chick health in subtle ways, its impact may last for generations to come.

"So now we see this really striking drop-off in numbers of birds as well as numbers of species of birds," he explained. "Both the biodiversity and the abundance are showing dramatic impacts in these areas with higher radiation levels, even as the levels are declining."

For more great nature science stories and general news, please visit our sister site, Headlines and Global News (HNGN).

- follow Brian on Twitter @BS_ButNoBS