Brightly coloured birds have been some of the hardest hit by radioactive fallout from the Chernobyl nuclear disaster, say ecologists.
High levels of radiation have also been found to have a particularly harmful effect on species that lay large eggs, or migrate long distances, say the researchers in the latest Journal of Applied Ecology.
"The research shows that the effect [of radiation] on birds is much stronger than previously thought – the open question is why it took 20 year before anybody bothered to look," said Anders Møller of the Pierre and Marie Curie University in Paris, France.
Formerly part of the USSR, the northern Ukraine city became the site of the world's worst nuclear accident when a reactor exploded in 1986. A plume of radioactive debris drifted over much of Europe and the Western Soviet union, resulting in the evacuation of 336,000 people.
Møller and co-author Timothy Mousseau of the University of South Carolina in Columbia, U.S., looked at 1,570 birds from 57 different species in and around Chernobyl, to examine the effect of radiation on local birdlife. The pair found that brightly coloured birds, whose red, yellow and orange plumage are coloured with carotenoid pigments, declined more than other species.
Antioxidant chemicals – such as carotenoids and the vitamins A and E – are the key, they argue.
Our bodies synthesise or collect these chemicals from food and use them to neutralise damaging free radicals, which are highly reactive metabolic by-products. Antioxidants also play a role in stimulating the immune system.
Radiation damage creates massive amounts of free radicals, which antioxidants help to mop up, said Møller, "animals have dramatically reduced antioxidant levels when exposed to radiation."
Therefore "we predicted that bird species that use large amounts of antioxidants – because of their peculiar life histories – would be most affected by radiation," said Møller. "And that was indeed the case."
Birds with brightly coloured skin, beaks or feathers - such as blackbirds, blue tits and orioles - get the pigments from carotenoid antioxidants, said Møller, and birds that lay large eggs, invest much of their supply of antioxidants in those eggs, for use by developing embryos.
Similarly, birds that use a lot of energy migrating or dispersing over great distances ¬¬– such as quails, hoopoes and orioles – generate many metabolic free radicals that need to be mopped up, so also have low antioxidant levels.
"Lack of bird song"
"Research into the importance of carotenoids on the health and viability of wild bird populations has produced fascinating results in recent years," commented zoologist John Ewen at London's Institute of Zoology in England. "There is now little doubt that these biochemicals play essential roles in maintaining health."
"The patterns presented indicate that radiation contamination affects population abundance and that the impacts are not evenly spread across species," added Ewen who was not involved in the research.
In general, abundance of bird populations was diminished by over 50 per cent in the most contaminated areas of Chernobyl, found the researchers, with species richness reduced by over two thirds compared to the least contaminated areas. "I was struck by the lack of bird song in contaminated areas," said Møller.
Møller and Mousseau published another study in March 2007 (Proceedings of the Royal Society B) showing that birds near Chernobyl prefer to nest in sites with lower levels of radioactive contamination – but they were unable to determine how the birds detected the difference.