Growing colonies of pigeons in empty buildings such as the Cathedral are proving a potentially lethal problem for Christchurch pilots.
The route the birds fly each day between the central city buildings and the Waimakariri River takes them directly over the airport, and there have been several near misses with planes.
Hitting pigeons causes damage to hundreds of aircraft each year, with the worst the crash of Ethiopian Airlines flight 604 in 1988, after it collided with a flock of pigeons during take-off. Of 104 people on board the flight, 35 were killed in the disaster.
The pigeons are also causing big problems for building owners, farmers and native birds.
Keystone Ecology ornithologist Niall Mugan said the pigeon population in Christchurch had grown dramatically over the past two or three years.
He had counted about 3000 of the birds gathered near the Waimakariri River.
Before the earthquakes, he said smaller numbers of pigeons roosted in the cliffs around Sumner.
But since large numbers had settled in empty buildings in the central city, he said the population had exploded.
"Judging by the amount of food available I would expect them to continue to breed and the numbers to continue to explode," he said.
Christchurch International Airport land use senior planner Kate McKenzie said the airport had a lot of techniques to discourage birds from landing there, but it was very difficult to discourage them from flying overhead.
Central city developer Ernest Duval said the birds were becoming a big problem for central city building owners, especially around the Christ Church Cathedral.
"It's the biggest aviary in the southern hemisphere," he said.
Riccarton Bush ranger Gavin Rucklidge said the high numbers of pigeons was causing big problems for native birds there.
One of the reasons they no longer kept kiwis at the bush was that the pigeon droppings were making them sick, he said.
"They are quite aggressive, stealing all the roosting sites and bullying the native birds," he said.
A co-ordinated approach with Environment Canterbury or the city council was needed to solve the problem, he said.
But ECan biodiversity and biosecurity regional manager Graham Sullivan said the birds had not officially been labelled a pest.
"The community would need to ask us to classify pigeons as a pest in the Regional Pest Management Plan. The plan is currently under review and we've had no such feedback so far," he said.