Birds dropping dead from the skies and rivers flowing with tens of thousands of dead fish sounds like a cheesy Hollywood movie about the Apocalypse. Or the ravings of a Revelation-obsessed street preacher.

But residents of several US states are coping with the reality of mystery mass wildlife deaths, which have left officials scratching their heads and jumpy members of the public joking (nervously) about the end of the world.

Today it emerged that about 500 red-winged blackbirds and starlings had been found dead in Louisiana. Their tiny corpses littered a short stretch of highway near the city of Labarre after apparently falling dead from the sky.

That would be spooky enough. But the Louisiana bird die-off came just a few days after up to 5,000 blackbirds fell to earth in neighbouring Arkansas in the small town of Beebe. Residents there had reported stumbling upon the bodies littering the ground and even being hit by them as they fell. One woman said she was struck while walking a dog. Another avian corpse bounced off a police car.

In even more grim news, anglers and other members of the public reported that more than 80,000 drum fish had suddenly died in the state's Arkansas river, about 100 miles west of Beebe. The silvery bodies of the fish floated in the river and washed up on its sides having died at roughly the same time. In another incident, hundreds of miles away on the Maryland coast of Chesapeake Bay, tens of thousands of dead fish also washed up on the shore.

Not surprisingly the news has startled and alarmed local residents and wildlife lovers across the US. But officials say such mass deaths do occur naturally.

The fish deaths in Arkansas appear to have affected only one species, which suggests the cause was a sudden outbreak of a specific disease. Meanwhile, the Maryland fish die-off was blamed on extreme temperature fluctuations due to cold weather.

The birds' deaths, however, are a deeper mystery. Officials were still collecting bodies in Louisiana but have already examined those from the incident in Arkansas.

They concluded that the birds had suffered internal trauma. That could have happened if a single flock had suddenly got caught in a violent and unusual storm. Or, it has been speculated, a local fireworks display could have startled the birds so badly that they were unable to prevent themselves from flying into trees, pylons and houses.

Bird experts stressed no one should be worried. "Mass bird die-offs can be caused by starvation, storms, disease, pesticide, collision with man-made structures or human disturbance ... Initial findings indicate that these are isolated incidents that were probably caused by disturbance and disorientation," Greg Butcher, director of bird conservation at the Audubon Society, said.

Audubon officials stressed that birds as a whole faced a far greater threat from broader environmental problems than any headline-generating mass death incident. "Far more concerning in the long term are the myriad other threats birds face from widespread habitat destruction and global climate change," Melanie Driscoll, Audubon's director of bird conservation in the Mississippi river region, said.

Louisiana's state wildlife veterinarian said yesterday at least some of an estimated 450 birds that died near Baton Rouge may have flown into a power line.

Jim LaCour said that the grackles, starlings, brown-headed cowbirds and red-winged blackbirds had broken beaks and backs.