Mercury poisoning could wipe out birds - by making them gay, say scientists.
A three-year study of 120 white ibises found more than half of males exposed to high levels of the chemical chose to pair off amongst themselves instead of with females.
Males dosed up with methylmercury, the organic and most toxic form of mercury, showed reduced rates of traditional courtship behaviour. Even relatively low levels of mercury in the diet of male white ibises cause the birds to mate with each other rather than with females.
As a result many of the females can't breed, and fewer chicks are produced.
Dr Peter Frederick, of the University of Florida, warned that gay coupling could result in a 50 per cent reduction in chicks. He said: 'These effects on reproductive behaviour and sexual preference represent a novel mechanism by which contaminants may influence wild bird populations.'
Mercury is a naturally occurring metal released into the environment from rocks and soils, and in volcanic eruptions. But human activities are continually adding more. Dr Frederick's team, whose findings are published in Proceedings of the Royal Society B, said methyl mercury is known to affect sex hormones of many animals but it is unclear whether bird populations may be affected.
They said: 'Male - male pairing contributed a large proportion of the reproductive deficits documented in this study, yet to our knowledge, this mechanism has not been reported as an effect of mercury exposure or of other contaminants. Homosexual behaviour among birds and other animals has been documented before but usually when there is a shortage of female birds in the local population.
But in this study, the researchers found that the homosexual pairing began when there were still plenty of potential female birds available. The paper says: 'In a wild colony of white ibises with minimal exposure to mercury, there were no male - male pairings observed in 134 white ibis pairs studied over 15,580 pair hours of observation during four breeding seasons.'
It is unknown whether mercury could have the same effect on humans. White ibises are long-legged, long-necked wading birds with short tails that nest in huge colonies in fresh water marshes or along the ocean coast.
Up to 80,000 individuals have been counted in one colony in the Everglades National Park, Florida. During the day they may fly at least 15 miles to find small crustaceans, fish, frogs and aquatic insects to eat and to feed their young.
The researchers said sexual display behaviour in birds is strongly influenced by circulating hormones and mercury exposure was associated with 'a de-masculinised pattern of oestradiol and testosterone expression in males, especially during courtship'.