Inspectors searching for the cause of the collapse of the Minnesota bridge on August 1, 2007, that resulted in 13 deaths, have identified pigeon droppings as a possible factor.

Structural engineers think that the build up of pigeon droppings over many years have sped up the rusting of the steel girders in the eight-lane bridge. The droppings contain ammonia and salt, which when combined with rain water, can lead to electrochemical reactions that rust steel. Over the long term this could lead to structural weaknesses.

Feral pigeons, which are descendants of cliff dwelling wild pigeons, are attracted to the flat surfaces of bridges and tall buildings for roosting and nesting. Dave Jones, Director of Deter-a-Pigeon, a UK based pigeon control company that uses pigeon spikes which safely deter pigeons without harming them, says the statement is ridiculous and stinks of a cover- up. He says "Inspectors began documenting the build- up of pigeon droppings on the span near downtown Minneapolis over two decades ago. Therefore, for over 20 years, bridge inspectors have sat back and watched pigeons roosting on the bridge. It is these inspectors who should have spotted any corrosion and acted accordingly with effective pigeon control." Jones presented an interesting analogy when he likened it to someone's teeth falling out because they have not brushed them for 20 years and then blaming plaque.

The problem of bird droppings is just one of the factors that inspectors are considering as they seek to prevent a repeat of the catastrophe in other ageing infrastructure. About 77,000 bridges across the US have been identified as having the same level of structural deficiencies as the Minnesota bridge.