From our good friends at the Guardian.
Guano theory in bridge collapse
Ed Pilkington in New York
Friday August 24, 2007
The GuardianInspectors searching for the causes of the collapse of a Minnesota bridge this month that killed at least 13 people have identified pigeon droppings as a possible factor.
It is thought the build-up of guano over many years could have speeded up the rusting of the steel beams in the eight-lane bridge, which collapsed into the Mississippi on August 1.
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.