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Alexandra's wild pigeon population has been reduced by 401, but each bird on average cost about $12 to kill.
The New Zealand Transport Agency (NZTA) has waged war on the birds roosting on the Alexandra bridge and the old bridge piers, saying the birds' excrement was corroding the bridge's structural steel, damaging the historic piers and posed a health risk.
The battle began in July, but the first attempt to kill the birds, using narcotic-laced bait, had failed. A different approach was taken in the next operation, which started last month. Hunters armed with air rifles, silencers and night-vision equipment were employed but were initially foiled by the moonlight, NZTA Central Otago manager John Jarvis said.
It was too light, making the hunters visible to their intended prey.
They had more luck in the past two or three weeks when, Mr Jarvis said, ''two marksmen were involved, along with a couple of spotters, and they shot the birds in the early hours of the morning ... The pigeons don't really move or fly about in the dark.''
''They successfully eliminated 401 and while that doesn't completely eliminate the problem, it made a significant dent in the population and brings it under control for now.''
The agency worked with police to ensure the safety of the operation. The hunters were based underneath the deck of the bridge. It was done in a controlled way, with people on top in radio contact with the hunters, and shooting being suspended if there were any traffic or pedestrians around.
The dead birds mostly ''sank in the water and most were probably eaten by eels'', he said. Any that fell on the river bank were removed and after each hunting session people in boats removed any floating dead pigeons from the river. The total cost of the culling efforts was about $5000, including $2500 for the night-shooting, Mr Jarvis said. Although similar operations were carried out in other areas, few were in such a public place ''right in the middle of a town''.
The battle was not yet over, Mr Jarvis said.
''In the new year we'll have to look at ongoing management options from here on in and this is likely to involve some more shooting. This is obviously a popular roosting spot, so birds will travel here to roost.''
''Now we'll bring some contractors in to clean up the areas of the bridge where the pigeons have been nesting and get up there and clear it out.''