ST. PAUL, Minn. – Pounded and strained by heavy traffic and weakened by missing bolts and cracking steel, the failed interstate 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 Interstate 35W 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.

“There is a coating of pigeon dung on steel with nest and heavy buildup on the inside hollow box sections,” inspectors wrote in a 1987-1989 report.

In 1996, screens were installed over openings in the bridge’s beams to keep pigeons from nesting there, but that didn’t prevent the buildup 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. 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.

“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,” said Langerman. “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. “Let’s let the highway transportation and safety people do their job,” he said.

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 to clean bridges wearing full biohazard suits with respirators because of heightened fears about 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 netting to block holes and surfaces, spikes to keep them from landing, and sometimes 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 and tall buildings since they’re descended from cliff-dwellers, said Karen Purcell, who heads Project PigeonWatch at the Cornell Lab of Ornithology. Bridges offer shelter from predators and flat surfaces for nesting.