ST. PAUL, Minn. &#8212 Besides such obvious threats as heavy traffic, missing bolts and cracking steel, the failed bridge over the Mississippi River faced a less obvious enemy – pigeons.

Inspectors began documenting the build up of pigeon dung on the span near downtown Minneapolis two decades ago.

Experts say the corrosive guano deposited all over the Interstate 35W span’s framework helped the steel beams rust.

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 problems that dogged the structure.

“There is a coating of pigeon dung on steel with nest and heavy buildup on the inside hollow box sections,” inspectors wrote in a 1987-89 report.

In 1996, screens were installed over openings in the bridge’s beams to keep pigeons from nesting in them, but that didn’t prevent the accumulation of droppings elsewhere.

Pigeon droppings contain ammonia and acids, said chemist Neal Langerman, an officer with the health and safety division of the American Chemical Society. Over time, the dung dries out and turns into a concentrated salt. When water combines with the salt and ammonia, it causes small electrochemical reactions that rust the steel underneath.

“Every time you get a little bit of moisture there, you wind up having a little bit of electrochemistry occurring and you wind up with corrosion,” Langerman said. “Over a long term, it might in fact cause structural weaknesses.”

Langerman emphasized that he wasn’t saying pigeon dung factored into the collapse of the 40-year-old bridge.

The problem is familiar to bridge inspectors everywhere.

The Colorado Department of Transportation spent so much time cleaning pigeon manure off bridges that it is embarking on a two-year research project looking for ways to keep pigeons away from its spans.

“It can be damaging to our structures because it’s slightly acidic and it has other compounds in it that can dissolve especially things like concrete,” said Patricia Martinek, the agency’s environmental research manager.

Pigeon guano isn’t just a danger to the bridges.

In the Denver area, the Colorado DOT pays outside environmental specialists, clad in full biohazard suits with respirators, to clean bridges due to heightened fears over bird flu and other diseases, said Rob Haines, who supervises maintenance there.

Keeping pigeons off bridges usually requires a multi-pronged strategy that can include placing netting and spikes on the bridges and poisoning, shooting or trapping the birds, said John Hart, a Grand Rapids, Minn.-based wildlife biologist with the U.S. Department of Agriculture.

The problem is that pigeons are naturally drawn to bridges, said Karen Purcell, who heads Project PigeonWatch at the Cornell Lab of Ornithology.

The span is scheduled for replacement in 2009. The nearest alternate bridge is 17 miles away.