LONGTIME ENEMY OF BRIDGES, PIGEONS TOOK TOLL ON FAILED MINNESOTA FREEWAY SPAN

Associated Press - August 21, 2007 1:14 PM ET

ST. PAUL (AP) - Investigators haven't yet determined what caused the Interstate 35W bridge to collapse.

The bridge was pounded by heavy trucks, weakened by missing bolts and strained by cracking steel. It also faced a more mundane enemy -- Birds.

The pigeons that made the bridge their home also used it as their toilet. Inspectors began documenting the buildup of pigeon dung on the 20 years ago. Experts say the manure they deposited all over steel beams helped rust the bridge faster.

A 1987-1989 report found a coating of pigeon dung on steel with nest and heavy buildup on the inside of hollow box sections of the bridge.

In 1996, screens were put over openings in the bridge's beams to keep pigeons from nesting there. Even so, the pigeon droppings continued to show up in later reports.