Thousands of red-winged blackbirds, cowbirds, starlings, and grackles dead in Arkansas. Five hundred more in Louisiana. Fifty jackdaws fall on a street in Stockholm. And around the world, millions of fish floating belly-up.
It's the stuff of apocalyptic novels. Scientists have not yet ruled out pollution or chemical toxins as the cause of nearly a dozen mass animal die-offs, from Arkansas to Brazil, in the last week. But as officials investigate, both the mundane and the intriguing are emerging as potential causes.
Because birds are considered indicator species that reflect the health of the surrounding environment, the spate of mass deaths has unsettled many Americans.
Explanations offered thus far "sound interesting and scientific, but may we remind you, birds have been flying our skies since the dawn of time and rarely have you heard reports of large quantities of birds just dropping to their death," writes Christopher Koulouris on the Huffington Post website. "Which leads one to wonder is there some fundamental shift in our ecology going on or is this just too much of a series of fluke random events?"
For one, an early cold-weather snap – one of the deepest in decades – may have played a role in the deaths of 50 birds in Stockholm and the deaths of millions of spot and croakers in the Chesapeake Bay on Tuesday, scientists say.
The 500 red-winged blackbirds and starlings that died in Louisana on Monday may have run into power lines, and fireworks set off by New Year's Eve revelers may have spooked a flock of 5,000 blackbirds that rained down on Beebe, Ark. Many of the birds had broken bones, indicating that they collided with stationary objects, probably after getting spooked, according to Arkansas wildlife officials.
But those findings aren't definitive, and some experts are pushing back at the fireworks explanation.
More likely, says Ed Clark, president of the Wildlife Center of Virginia, is that the Arkansas birds were targeted by a farmer fed up with them feeding on his cattle grain. No licenses are needed to kill blackbirds and starlings, and poisoning is a common method of controlling large, noisy flocks.
"This was a deliberate act of avicide, legal or illegal," writes Mr. Clark in a comment on the Huffington Post. (Surface tests on the birds turned up no evidence of poison, but it's not yet clear whether the birds ate something poisonous.)
Mass bird kills aren't uncommon. The US Geological Survey documented 90 mass deaths of birds from June to December last year. Over the past 30 years, it counts 16 events in which 1,000 birds or more suddenly died.
Dan Scheiman of Audubon Arkansas speculates that many more such events happen than humans see, because scavengers quickly dispose of the evidence.
Testing can take time and is often inconclusive, although methods have improved in recent years, says Greg Butcher, a conservationist at the National Audubon Society. Scientists hope to have an explanation for the Arkansas bird kill within three weeks.
Fish, too, are susceptible to environmental stress, including extreme cold. The cause of some 2 million spot and croakers going belly-up on the Chesapeake Bay in Maryland is probably linked to an uncommonly cold early winter, biologists there say. Similar fish kills were observed in the bay in 1976 and 1980. The past decade – not to mention the past month – has been one of the snowiest and coldest in many parts of the US, including the South.
Millions of fish die in kill-offs every year in the US, with causes ranging from pollution to stress. Moreover, some 5 billion birds die annually, most from natural causes.
Nevertheless, officials in Arkansas and Louisiana call the large number of bird deaths "unusual." While the ultimate explanations may not point to broader environmental problems, "it is something we should potentially worry about," says Mr. Butcher at the National Audubon Society.