But their most recent findings, published last month, showed something new. Some bird species, they reported in the journal Functional Ecology, appear to have adapted to the radioactive environment by producing higher levels of protective antioxidants, with correspondingly less genetic damage. For these birds, Dr. Mousseau said, chronic exposure to radiation appears to be a kind of “unnatural selection” driving evolutionary change.

Ionizing radiation, like that produced by cesium, strontium and other radioactive isotopes, affects living tissue in several ways, among them by breaking strands of DNA. A high enough dose — many thousands of times higher than the levels in the forest — can cause sickness or death. That is what happened to several dozen technicians and firefighters at the Chernobyl plant when the Unit 4 reactor exploded on April 26, 1986. They were exposed to lethal doses, in many cases in just a few minutes, and their organs and tissues were so badly damaged that they died within weeks.

Relatively low doses of radiation, however, even over a long time, may have little or no effect. But lower doses can cause genetic mutations, leading to cancers and other physical problems that may show up over longer periods and affect breeding and longevity. Studying the effects on animals and insects can lead to a better understanding of the impact on people as well.

Some researchers have challenged the studies by Dr. Mousseau and his collaborators, arguing that it is difficult to show that radiation levels in the exclusion zone, which covers about 1,000 square miles, have had much noticeable effect. There have also been anecdotal reports of abundant populations of some animals in the zone, suggesting that the lack of human activity there has led to the area becoming a haven for wildlife.