A five year study by the University of Florida shows that a diet containing relatively small amounts of mercury causes male ibises to nest together. Since mercury finds many ways into our environment, this can negatively impact the number of young ibises hatched in the wild.
This makes me wonder about other species of birds as well. In particular, I wonder about the San Francisco bay area, my hometown, because the bay contains high levels of mercury due to gold miners using mercury to separate the gold from the rock. Could this be altering the behavior of our birds? And what about the other wildlife? Me thinks this needs more investigation.
A note: This in no way implies anything at all about human sexual orientation. Humans physiology is dramatically different from the physiology of birds and studies on the effects of mercury on humans found no change in sexual behavior.