This would be strange if it turns out to be true:

Pounded and strained by heavy traffic and weakened by missing bolts and cracking steel, the failed Interstate 35W bridge over the Mississippi River also faced a less obvious enemy: Birds, specifically pigeons.
Inspectors began documenting the buildup of pigeon dung on the span near downtown Minneapolis two decades ago.
Experts say the corrosive guano deposited all over the span’s framework helped the steel beams rust faster.
Although investigators have yet to identify the cause of the bridge’s Aug. 1 collapse, which killed at least 13 people and injured about 100, the pigeon problem is one of many factors that dogged the structure.
[…]
Pigeon droppings contain ammonia and acids, said chemist Neal Langerman, an officer with the health and safety division of the American Chemical Society. If the dung 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.