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It has long been a mystery why agile birds cannot out-manoeuvre huge jetliners, causing aircraft to make emergency landings in extreme circumstances.

But now scientists have worked out why, and it appears that birds don't take into account an aircraft's speed, when calculating how to avoid it.

This failing means that birds have little time to escape vehicles such as aircraft and speeding cars, which travel faster than 75 mph (120 kph).

Scientists believe that birds don't take into account an aircraft's speed, when calculating how to avoid it, instead solely relying on how far away the aircraft is from them at a certain time (stock image). This results in the deaths of thousands of birds every year and potential aircraft disasters

According to the Federal Aviation Administration (FAA) 9,000 birds are struck by aircraft over the US every year and the estimate is conservative, because bird strikes don't have to be reported.

Bird strikes not only usually kill the animals, but can injure drivers if they hit a vehicle's windscreen, as well as posing a potentially serious threat to flight safety, because they can cause engine failure.

To work out why the strikes happen, a team of scientists from the US Department of Agriculture's National Wildlife Research Centre in Ohio, Indiana State University and Purdue University, also in Indiana, used virtual reality, to avoid injuring birds.

Researchers found that the birds they studied left it too late to escape vehicles travelling faster than 75 mph (120kph). Essentially, they judged a safe distance to manuoeure without calculating that the vehicles might approach them at speed, as shown in this diagram

The experiment took place in an enclosed chamber where brown-headed cowbirds were played videos of lorries rushing towards them at speeds ranging from 37 to 224 mph (60 to 360 kph), which is the cruising speed of some small airplanes and falls within the take-off speed for commercial aircraft.

By measuring their reactions, the scientists discovered that the birds seemed to react based on the distance between their bodies and the lorry, ignoring how fast the vehicle was travelling.

The birds repeatedly started to fly away from the vehicle when it was 98 ft (30 metres) away, according to the study, published in the Proceedings of the Royal Society B.

The study says: 'Alert and flight initiation distances remained similar across vehicle speeds, and accordingly, alert and flight initiation times decreased at higher vehicle speeds.

'Thus, avoidance behaviours in cowbirds appeared to be based on distance rather than time available for escape, particularly at 60–150 km.'

While the strategy worked for avoiding slower moving objects, the researchers observed that it failed when trying to escape vehicles travelling faster than 75mph (120kph), such as cars, lorries and airplanes.

They were unable to pinpoint exactly how the birds reacted to speeds above 112 mph (180kph) – cruising speeds typical of a small aircraft, or commercial aircraft at take-off.

The scientists concluded: 'As vehicle speed increased, cowbirds did not have enough time to assess the approaching vehicle.

'Although potentially effective for evading predators, the decision-making process used by cowbirds in our study appears maladaptive in the context of avoiding fast-moving vehicles.'

They said that further research is needed to check that different species of birds use the same avoidance techniques and suggested that installing lights on aircraft could help avians dodge death from a longer distance away.

The scientists said that further research is needed to check that different species of birds use the same avoidance techniques as brown-headed cowbirds (stock image). They also suggested that installing lights on aircraft could help avians dodge death from a longer distance away