According to Neal Langerman, of the American Chemical Society, pigeon droppings contain ammonia and salt which, when combined with rainwater, lead to electrochemical reactions that rust steel and cause serious structural weaknesses. Mr. Langerman noted that if the exrement isn't washed away, it dries out and turns into a concentrated salt. When water gets in and combines with the salt and ammonia, it creates small electrochemical reactions that rust the steel underneath.
Pigeon Excrement is thought to be one of the contributing factors in the collapse of Bridge 35W in Minneapolis, Minnesota on August 01st, 2007 - killing 13 people.
It is believed the build-up of bird excrement over many years sped up the rusting of the steel beams in the eight-lane bridge.
Structural engineers had been aware of the problem since as early as 1987, when inspectors noted a coating of guano on the inside of some of the steel girders. In 1996, screens were installed over openings in the bridge's beams to keep pigeons from nesting there, but that did not prevent the building of droppings elsewhere.
Bridge inspectors everywhere are aware of this problem: The Colorado Department of Transportation spent so much time cleaning pigeon droppings off bridges that a two-year research project has been launched, looking for ways to keep pigeons away.
"Pigeon Droppings are damaging to our structures because they are acidic and have other compounds that can dissolve especially things like concrete," said Patricia Martinek, the agency's environmental research manager.
With its large quantities of bacteria and highly corrosive uric acid, pigeon excrement is partly responsible for disasters such as this bridge collapse, as well as the damage of concrete and countless historical stone structures.
According to Venice's department of fine arts and historic monuments, pigeons cause the most damage "to plaster and stucco used on the exterior of buildings and the mortar used in restoration work".