Minnesota: Pigeon droppings have been identified as a possible factor in the collapse of a bridge over the Mississippi river that killed 13 people and injured a further 100.
The droppings contain ammonia and acids. They dry to leave a salt which, when combined with moisture, accelerates the corrosion process. Keeping bridges clear of the droppings is a major consideration for transportation departments.
Other factors being considered as possible contributors are cracked steel and missing bolts. The de-icing system is also being checked to determine if it uses any corrosive chemicals.