Determinants of interspecific variation in population declines of birds after exposure to radiation at Chernobyl

Radiation can reduce antioxidant levels dramatically because of the use of antioxidants to eliminate free radicals produced in the presence of radiation. Antioxidants are crucial biochemicals for elimination of free radicals, which can cause permanent damage to DNA and other molecules. If antioxidants are a limiting resource, we would expect individuals of species with a high expenditure of antioxidants to suffer the most from radiation. We tested this hypothesis by investigating interspecific variation in the relationship between abundance and level of radiation in breeding birds inhabiting forests around Chernobyl, Ukraine. We used bird point counts to estimate abundance of 57 species of birds at 254 locations where background radiation levels were quantified. Migratory birds use large amounts of antioxidants during their annual migrations to neutralize free radicals, and migrants have depleted antioxidant levels upon arrival at their breeding grounds. Consistently, abundance decreased with increasing levels of radiation in species that migrated the longest distances. Bird species with long dispersal distances may experience deficiencies in antioxidant levels because of physical activity but also because of exposure to novel antigens, implying that species with long dispersal distances should suffer the most from exposure to radiation. Indeed, the slope of the relationship between abundance and radiation decreased with increasing dispersal distance. Female birds deposit large amounts of antioxidants in their eggs, with the total amount deposited often exceeding the total amount in a female's. Accordingly, the decrease in abundance with radiation level increased with relative egg size in different species. Many bird species have plumage that is coloured by carotenoids, which cannot be recovered once deposited in feathers. Therefore, bird species with carotenoid-based plumage should show stronger declines with increasing levels of radiation than species with melanin-based or structural coloration. In accordance with this prediction, the decline in abundance with radiation was the strongest in species of birds with carotenoid-based plumage. Synthesis and applications. These findings highlight the importance of antioxidants for understanding the ecological consequences of radiation on the abundance of free-living animals, showing that species using large amounts of antioxidants will be particularly susceptible to the effects of low-level radiation.