The excrement of avians is not being ruled out as a factor in the collapse of the I-35 bridge on the River Mississippi in the Twin Cities. Apparently, pigeon dung, when dried, exudes salts. Salts which corrode steel and wear away concrete. Years back, grates were installed on the bridge's structure to discourage nesting, but they didn't necessarily stop the rampant pooping of birds with an interest in a river view. While authorities admit that it's far from certain that bird shit caused the July collapse, they're looking into the possibility. The lingering spirit of Alec Guinness, at the present time, is not under suspicion. [SFGate]