Of all the possible causes of the deadly collapse of Minnesota's Interstate 35W bridge earlier this month--uneven traffic patterns, de-icing salts, faulty construction--the latest is the most surprising: Pigeons. Or more precisely, the waste the birds leave behind. "Pigeon dung can be a serious issue--it's acidic and will easily eat away almost any metal," explains engineer William Schutt, president of Matcor, a corrosion-protection firm in Doylestown, Pa. "It can wash into and then rust the bolts and rivets of bridges if they're not cleaned and checked properly."
The build-up of pigeon excrement on the I-35W bridge was substantial enough to be noted in several Minnesota Department of Transportation inspections over the years, pointing to the steel box sections of the bridge as a popular nesting spot. Those sections are crucial to supporting the structure, and in 1999 bridge workers placed plastic screens over openings in the beams in an effort to repel the birds. But the dung continued to pile up. A 2006 inspection of the bridge still reported "severe pigeon debris" on its steel deck truss.
Corrosive pigeon dung also made it hard for bridge inspectors to do their work. Three experts familiar with the I-35W bridge told the Minneapolis Star Tribune that some of the impediments to inspections of the collapsed bridge included piles of pigeon dung, poor lighting, road rage (commuters upset with delays were known to insult inspectors and even occasionally throw objects at them) and spider webs that could resemble metal cracks.
Schutt points out that manure, in general, needs to be handled carefully because of its acidity. "For example, when cow manure for fertilizer is shipped or stored, it is usually held in glass-lined containers." As for bridges, he argues, a simpler solution is to find ways to drive away the birds or at least wash away the corrosive dung.
Could pigeon dung alone be responsible for the catastrophe? "We can't speculate too much; the investigation is still going on and this is early in process," says Neal Langerman, a health and safety division expert with the American Chemical Society. But he says there is "a chemical mechanism by which the high salt levels of dried pigeon dung can cause corrosion and weaken metal." That said, pigeon droppings, he argues, couldn't have altered the bridge's structure to a significant degree. "Pigeon dung might ruin a bolt or a plate here or there, which could potentially cause the bridge to sag somewhat. It might be found to be a contributing factor but probably not a root cause of the collapse."
America's bridges and skyscrapers are a "spectacular" substitute for the native range of pigeons--the cliffs along the Mediterranean--explains John Fitzpatrick, director of the Cornell University Lab of Ornithology (which is home to a pigeon education program called Project Pigeon Watch. Once they've reached their lofty roost, pigeons do what they're best known for, at least among those of us who must live on the ground beneath them.
But there are mysteries to the pigeon that few of us know. "Pigeons don't pee," explains Fitzpatrick. "It's a water-saving mechanism. Reptiles and birds crystallize their ammonium waste into uric acid, which is the white stuff in bird poop. It forms a salt when it dries and could certainly accelerate corrosion of steel."