In the April issue of Fortune Magazine, the article "Hysteria About Acid Rain" appeared, written by William M. Brown. In the article the author points to bird droppings as a bigger contributor to soil acidification than industrial sulfur dioxide emissions.
"The amount of acid generated by nature is now known to be far greater than that contributed by industrially generated acid rain. Take bird droppings, which are a relatively minor contributor to the problem. A calculation based on Audubon Society data shows that the droppings hit the U.S. at a rate of about one million per second, and the 150 million tons of droppings per year outweigh sulfur dioxide emissions by something like six to one." (Brown, 1986)
Brown based his argument on a study done by the Hudson Institute, an Indiana group funded by the coal industry and electric utilities. Brown himself was associated with the Cato Institute (Oreskes and Conway, 2010, p.101)
Source: http://archive.fortune.com/magazines/fortune/fortune_archive/1986/04/14/67366/index.htm