In recent years, there have been reports that the area around the Chernobyl nuclear plant had become something of a wildlife playground. The reports suggested that animals like wild boar, wolves and moose had flourished in the 40-mile-diameter “exclusion zone,” which was contaminated by low-level radiation from the disaster in Ukraine 21 years ago.

But Timothy A. Mousseau, a biologist at the University of South Carolina and co-director of the university’s Chernobyl research initiative, said there had not been systematic studies of wildlife there. “When we sat down to review the literature, we realized that most of these claims were just that — claims,” he said.

So Dr. Mousseau and his co-director, Anders P. Moller of Pierre and Marie Curie University in Paris, decided to systematically study the animals they know best: birds. Wearing a protective suit, Dr. Moller conducted simple bird counts throughout the zone. Radiation levels were measured at each site.

Close to 1,600 birds were counted, representing 57 species. But as the researchers report in Biology Letters, both the number of species and abundance of individual birds declined with increasing radiation levels. For example, the most contaminated sites had about two-thirds fewer birds than those with normal levels of radiation.