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Their quarry will be the feral pigeons whose dung and roosting activities are corroding the steel of the bridge over the Clutha River and damaging the nearby historic bridge piers.
Following an unsuccessful July offensive on the estimated 400-strong pigeon population, the New Zealand Transport Agency has turned to air rifles instead of poisoned bait.
Agency area manager John Jarvis said the birds did not seem interested in the bait and so the agency had to change tactics.
Following approval from relevant authorities including the police and SPCA, agency contractors will soon begin their operation, working in darkness below the bridge deck where most of the birds roost.
Mr Jarvis said it would be "a bit of a trial-and-error" process to begin with. It was likely only one person would be armed at any one time.
That person would be in radio contact with team members stationed at either end of the bridge who would control traffic while shooting was taking place.
While this method would be slightly more expensive than the poison option, it would have a higher success rate.
However, "I would be surprised if we got the whole lot", Mr Jarvis said.
But a reduction in numbers would allow the agency to clean excrement from the bridge to assess the damage.
Mr Jarvis said the excrement was corroding the structural steel of the bridge and while it was not an immediate safety risk, pigeon numbers needed to be reduced now so the bridge could be repaired.
The birds were last culled 25 years ago.
The work is expected to take several weeks.