Effects of Mercury on Ibis Behaviour

Source: text and photo, nationalgeographic.com
Mercury in the food of wild white ibises leads to changes in their behaviour. Even low mercury levels can have a substantial effect on reproductive behaviour, resulting in more homosexual pairs and fewer young.
The wild white ibis is one of the most common birds in the Florida (USA) Everglades. The birds are exposed to mercury through their food (shellfish and small invertebrates). Industrial processes such as waste incineration have introduced mercury into the Everglades environment. Though sources of pollution have now been reduced, the process has not yet been stopped entirely.
A large, five-year study followed 160 white ibis. The birds were assigned to four groups, each of which contained equal numbers of males and females. The animals were allowed to choose their own mates, which the researchers report is unique. The range of mercury levels in the experimental groups was comparable to those in the wild. The control group received a mercury-free diet.
There was an increased rate of homosexual bonding in the experimental groups. These male-male pairs did everything that a male-female pair would do: build a nest, copulate, and stay on the nest for a month, even though there were no eggs. This situation led to the presence of 13-15% fewer young compared to the control group. Even in the heterosexual couples, the mercury had a discernible effect: females bore 35% fewer young than in the control group.
Mercury disrupts or blocks the production of natural sex hormones called oestrogens. Male ibis that had consumed mercury had higher ratios of estrogen to testosterone as compared to the control group. Still, the effect of mercury on choice of mate is not clear. Also unknown is whether the ibis behaviour would change if the mercury would disappear from the environment.

The results of this study have been published in Proceedings of the Royal Society B.