Experts Tie Pigeon Dung, Bridge Collapse

Pigeons gather under the Robert Street bridge Friday, Aug. 17, 2007 in St. Paul, Minn., where they set up nests, offering shelter from the skies and predators above. Pigeons, a problem familiar to bridge inspectors everywhere, produce slightly acidic dung that can dissolve things like concrete and cause rusting on steel bridge beams. Pigeon dung is one of the things inspectors are looking at in the Interstate 35 bridge collapse in Minneapolis. (AP Photo/Jim Mone)

(AP) -- 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: pigeons.