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Some commentators have dubbed it "the aflockalypse" - instances of mass bird deaths that have perplexed observers over the past week. So why have birds quite literally been falling out the sky?

It all started when residents of Beebe, in the US state of Arkansas, woke to find thousands of dead blackbirds strewn across roads on 1 January. Then, in Louisiana, about 500 birds - including starlings, cowbirds and redwing blackbirds - rained down from the skies.

Only days later, news outlets once again became excited by the discovery of dozens of unfortunate jackdaws who had met their fate over Falkoeping, in Sweden.

Other bird and fish die-offs - as these mass deaths are called - were also reported as far away as Japan, Thailand, Brazil.

Conspiracy theorists have rushed to conclude that the apocalypse is nigh, while other people have mooted the idea of collisions with UFOs, or government testing of satellite-powered energy weapons.

But experts insist that what is going on is not that unusual and that the incidents are unconnected.

Such events are usually blamed on bad weather, disease and poisonings.

On Friday, Italian scientists were investigating the deaths of the 400 turtle doves, whose bodies appeared in the days after New Year. The initial focus of the investigation was on known diseases, epidemics and viruses.

In the case of the Arkansas blackbirds, it was initially thought that the deaths were caused by some weather phenomenon - a microburst, or strong downdraft of wind, perhaps.

But the die-off, which took place on New Year's Eve, was most likely caused by fireworks, local experts say.

Scientists from the Arkansas Game and Fish Commission said the red-winged blackbirds probably flew low to avoid the turn-of-the-year fireworks and collided with objects.

An investigation is continuing.

But Grahame Madge, conservation spokesman for the Royal Society for the Protection of Birds (RSPB), describes the Arkansas case as "bizarre and mysterious", and says that it would be "almost unique" in the UK.

He suggests that the birds may have in fact been poisoned, arguing that it was a strange coincidence that the mass deaths involved the red-wing blackbird, which is considered an "agricultural pest".

"It is intriguing that the bird at the epicentre of this particular incident is also the most hated," he says.

He says they are sometimes targeted with a poison that affects the water repellency of the birds. As a result, they get wet and cold and develop pneumonia.

"If they took flight and were experiencing extreme cold or suffering the effects of the poison, it is possible that that brought them down," he adds.

Arkansas officials said preliminary testing showed no sign of disease in the dead birds and that they died of "acute physical trauma".

But Mr Madge says this could have occurred when they came into contact with buildings or the ground.

Martin Fowlie from Birdlife International says the firework explanation is a plausible one.

"If they were roosting at dark, one could imagine large-scale fireworks could put them up in the air and they become disorientated and they fly into something or into the ground," he says.

"But I also heard some reports that some birds were acting drunkenly - which would imply poisoning."

Mr Fowlie said because die-offs usually occurred in smaller numbers, many of them go unreported.

It could be that the size of Arkansas incident, combined with the fact that it took place at the turn of a new year and was followed shortly after by another die-off, prompted wider reporting of the subsequent incidents, as people tried to join up the dots.

Kristen Schuler, a scientist at the US Geological Survey's National Wildlife Health Center, told the Associated Press that the incidents of the past week were "not that unusual".

"There is nothing apocalyptic or anything that is necessarily out of the ordinary for what we would see in any given week," she says.

The USGS tracks mass deaths among birds, fish and other creatures. It says they range in size from the dozens to the thousands.

The UN Environment Program (UNEP), however, has called for more research into animal deaths in general.

"Science is struggling to explain these things. These are examples of the surprises that nature can still bring," said Nick Nuttall, a spokesperson for UNEP.

"More research is needed."

The RSPB's Grahame Madge says there has been some degree of hysteria after this week's bird incidents.

"I think there are people trying to join the dots. There is a great deal of speculation about whether it is a global phenomenon.

"What we are dealing with is separate incidents, which have different reasons. We are not undergoing a mass wipe-out of birds."