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Passengers in an inflatable raft prepare to move away from the Airbus 320 that ditched in the Hudson River after a likely bird strike. ((Bebeto Matthews/Associated Press))

When a bird collides with an airplane, the result is often a grisly one for the bird. But its swan song can also include major damage to the plane.

Barely six minutes after takeoff from LaGuardia Airport in New York City on Jan. 15, 2009, US Airways Flight 1549 was headed for an emergency landing in the Hudson River — likely forced down, according to the U.S. Federal Aviation Administration, after bird strikes disabled both of the plane's engines.

All 155 people on board survived the crash, a credit to the pilot's skill and the aviation industry's years of experience dealing with plane-on-bird collisions.

A total of 1,324 bird strikes were reported to Transport Canada in 2007, the most recent year for which statistics are available.

Of the birds later identified, the most common were gulls, with 151 incidents in 2007, and sparrows, with 55 collisions.

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Damage from a bird strike test conducted at the NRC's Institute for Aerospace Research. ((NRC))

Here we look at the science behind keeping birds and planes out of each other's path, and what happens when they do meet.

How can a small bird bring down a plane?

With the exception of geese and swans, wild birds that typically strike planes weigh less than 3.6 kilograms, but depending on the speed of the plane during the collision, they can pack quite a wallop.

Birds are most likely to hit aircraft during takeoff or landing, as was the case with US Airways Flight 1549, and not at high altitudes, according to the National Research Council's Institute of Aerospace Research.

Collisions typically occur less than 1,500 metres above ground and near airports. Planes are mandated not to fly faster than 650 km/h when near airports, so that helps keep their speeds down should they hit a bird or flock of birds.

But at that speed, even small birds can do plenty of damage, according to Ron Gould, a technical officer at the institute in Ottawa. When calculating the kinetic energy of an object, the handy equation is one half of mass times velocity squared, says Gould.

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Sparrows were the second most likely bird, after gulls, to strike a plane in Canada in 2007. ((Associated Press/Vadim Ghirda))

So a two-kilogram bird flying into a plane at 650 km/h (or 180.1 metres/second) would hit the plane with a kinetic energy of 32,600 joules.

By comparison, a major league fastball packs about 112 joules, a bullet from a rifle is about 5,000 joules and a hand grenade packs about 600,000 joules.

Again, speed is key. A bird hitting a plane travelling half that speed would hit with a quarter the energy.

Gould also says soft objects don't hit quite the same way as a hard rigid object either: while a solid object will make a hole roughly equivalent to its size, a soft object like a bird in flight will spread on impact, potentially causing more damage.

What parts of a plane are vulnerable?

No plane or helicopter is "bird-proof," but most are designed to withstand a collision and still be able to land safely.

Of the over 1,300 collisions recorded in 2007, 1,081 of the flights continued with no effect, and only three needed to make a forced landing, according to Transport Canada.

What kind of damage a bird causes depends on where it hits, says Gould.

"If the bird strikes on the windshield, you are going to lose the optics. The laminated glass windows are going to crack, and that's why there is more than one window and more than one person looking out through the windows," he says.

Engine strikes are more problematic, he says, since a bird that passes through the turbine can get into the engine duct.

Transport Canada said while birds most often struck the windshield, wings or nose of the plane, damage was most likely to occur when the engine was hit, with about 30 per cent of all engine strikes resulting in damage.

Aircraft can fly with only one working engine, but in the case of US Airways Flight 1549, where multiple bird strikes disabled both engines, the pilot has few options other than an emergency landing, Gould said.

Sometimes pilots get lucky, too. Gould said there have been many eyewitness accounts of birds passing through an engine unscathed, beneficiaries of excellent timing as they pass through spinning engine blades.

How do engineers protect planes against bird strikes?

At the NRC, the weapon of choice is the "chicken cannon," which has been satirized on the CBC's Royal Canadian Air Farce for years.

The real chicken cannon, a 10-inch-bore, 23-metre-long gun, uses compressed air pressure to launch chickens at various airplane components to test their durability.

Since 1968, the "chicken cannon" has fired more than 3,500 times, consuming more than 3.5 tonnes of chickens in the process.

The NRC's institute uses deceased domestic egg-laying chickens for bird strike tests. They are kept frozen and then thawed to room temperature before being shot from the cannon.

During engine-ingestion tests, smaller guns are used to fire smaller birds.

When calibrating the cannon, the researchers use mock chickens made of gelatin and fibrous material, but to truly simulate the impact of a live bird, Gould said there is no substitute for the real thing.

Are other measures in place to keep birds away?

Making planes bird-resistant is actually the last resort, as far as flight safety organizations are concerned.

Airports use several methods to ensure birds stay away from the immediate vicinity, employing noisemakers and birds of prey, and keeping the surrounding grounds well-groomed to make them uninviting as a location for nests.