KEY POINTS:
Some New Zealand airports are struggling to control the risk of bird strike, despite keeping a constant eye on feathered visitors to their runways.
Two of the 18 airports monitored by the Civil Aviation Authority are considered high risk. According to June 2006 figures (the latest available), there are more than 10 bird strikes per 1000 aircraft movements at Gisborne and Invercargill. A further six airports are at medium risk, with more than five strikes, and have all been identified as having a growing problem.
Earlier this month, the problem was dramatically highlighted when a plane leaving Manchester Airport, in Britain, was forced to make an emergency landing due to an encounter with two herons.
The birds were sucked into one of the plane's engines, causing flames to appear before the pilot landed his 221 passengers safely back at the airport.
"Is it a major risk in NZ? I don't think it is. Is it a risk that's recognised? Yes, it definitely is. People are continually working on it," said CAA spokesman Bob Sommer.
In New Zealand, all incidents involving birds - whether near misses or direct hits on the side of an aircraft or in an engine - must be reported to the CAA by the pilot.
"They [airports] take action to try and reduce the risk. Each of them has its own plans to mitigate the risk - the most important thing is to recognise that [the risks] exist."
Gisborne Airport, with the highest rate of strikes at 11.5 in 1000, uses several methods to control wildlife within the airport but is struggling to deal with a high population of the native spur-winged plover, which is protected under the Wildlife Act.
"Overall numbers of these bird species appear to have increased significantly over the past couple of years, particularly within the extensive agricultural cropping and viticulture areas surrounding the airport," said Gisborne airport manager Murray Bell.
With the second highest rate of 11.4 strikes, Invercargill Airport also has a large number of spur-winged plovers. But its incident rates tended to be cyclical, said operations manager Eric Forsyth. "You can't always explain why you've got your numbers under control and why they then go up," he said.
The risk to passengers is most severe in single-engine aircraft, as most larger planes can function on one engine, if a bird damages the other.
Auckland Airport, New Zealand's largest, has a low bird-incident figure, with only 2.7 strikes per 1000.
But it takes plenty of work and the services of a fulltime wildlife hazard officer - "the birdman" - to keep it that way.
"Birdman" David Harris controls the seagulls, skylarks, dotterells and black swans around the airport.
Spur-winged plovers are also very common at Auckland, and Harris calls them "the baddies". No matter how much you learned, birds were unpredictable, he said.
Measures used to keep birds away include gas cannons that make loud bangs at certain intervals; metallic streamers that crackle in the wind to replicate fire; sirens and other noisemakers; bird kites that scare smaller birds away; and bird fright, a shot of ammunition into the air.
If birds are active in the area, a warning is given to pilots, who adjust their flight plans to avoid flocks.
The numbers of worms and invertebrates which birds eat are monitored; grass is cut to particular heights; water bodies where birds like to roost are reduced; and sometimes a hovercraft quietly moves along large groups of swans on the nearby harbour.
Bob Parkinson, operations manager at Auckland Airport, said the aim was to make it unpleasant for birds to stay there. Shooting birds and poisoning programmes were a last resort.
Auckland Airport works with other airports on wildlife management and recently advised Tonga and Samoa.
