The cost of repairing a bridge in North Wales has surged from the expected £844,000 (AU $1,557,175) to £1.53 million (AU $2,823,060) because the extent of the rust damage was hidden beneath a layer of pigeon poo.

Works to repair Conwy Road Bridge were sent out to tender in 2016, but a recent inspection revealed rust on the structure that had been hidden by droppings.

A report by Conwy Country Borough Council's audit committee said the "considerable" extra work needed had not been found originally because of "access" problems, according to the BBC.

"The additional works...significantly impacted the total cost of the works," the report said.

Council chief executive Iwan Davies said: "There was no negative. What they identified was work that needed to be done in any case.

"They did a survey of the bridge. It wasn't possible to identify the extent of the works because the corrosion to the steel work was actually hidden by lots of pigeon dirt, so what it identified was that the contract should have been for that bigger amount but earlier on we thought it was less.”

It’s not the first time avian excrement has created problems for engineers. Pigeon poop is acidic and corrosive, and cities all over the world spend big bucks making sure it doesn’t eat away at infrastructure assets.

The bridge as seen from Conwy Castle, with the modern road bridge to the left.

And, it's not just pigeons that are to blame. In 1923, engineers in charge of the Big Creek hydroelectric dams in the US were baffled by unexplained short circuits. The rate of disruptive flashovers – arcs of electric current that sometimes leapt from the wires to the steel towers and into the earth – jumped dramatically.

The culprit was found when a worker saw what appeared to be an eagle perched on top of a transmission tower. As the bird took to flight it let loose a long jet of droppings onto the power lines.

Hawks and eagles were apparently attracted to the transmission towers as high lookout points from which to hunt. They gave off streams of highly electrically conductive excrement as they launched from towers, which lab tests later confirmed could have caused flashovers by carrying current from the power lines to the steel towers even without touching either.

It was an almost perfect crime; the excrement trails were destroyed by the concentrated energy of the resulting electric arc, leaving no observable evidence behind. After the power company Southern California Edison installed barriers, spikes and excrement-catching pans on the towers, the number of short circuits dropped dramatically.