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Over the past two weeks, 500 red-wing blackbirds died together in Louisiana, while 100 jackdaws turned up dead on a street in Sweden. And in Arkansas, an estimated 100,000 fish went belly-up the day before 5000 blackbirds slammed into roofs, mailboxes and the ground at full speed.

Such massive, dramatic and high profile events have fuelled concerns that nature is coming to an end, or at least that something weird and disturbing is going on in the animal kingdom.

But according to wildlife health experts, major mortality events happen every year for reasons that include bad weather, disease outbreaks and poisonings. The main reason this recent spate of events seems so strange is that, most of the time, mass deaths occur in places where nobody notices them.

"This is really not the unusual thing that people are trying to make it into," says Robert Meese, an avian ecologist at the University of California, Davis. "A lot of this stuff happens without anyone documenting it."

Dramatic die-offs are most common in animals that congregate or travel in large groups.

Migratory birds, for example, often become disoriented by fog or storms, causing them to run into towers, bridges, wind turbines and trees. On their long journeys, migratory birds may also accidentally ingest pesticides or even poisons that were left specifically for them. Weather can work against them, too, particularly if they migrate too soon in an unseasonably cold year.

Records kept by the United States Geological Survey list at least 16 die-offs of more than 1000 blackbirds or starlings over the past 30 years, says Marisa Lubeck, a spokesperson for the USGS in Denver. But group deaths among animals have been going on for a lot longer than that.
History of mass deaths

In a review study published in 2007 in the journal Ibis , researchers looked through European and North American bird journals and other references dating back to the late 19th century. They found frequent reports of deaths of birds in the hundreds, thousands or more.

In one of the most extreme examples, an estimated 1.5 million Lapland Longspurs died during a March 1904 storm in Minnesota and Iowa.

And it's not just birds that can die en masse. Tens of thousands of salmon died in Northern California's Klamath River in 2002 when the water temperature got too high for them.

Strandings of whales, seals and turtles get occasional attention. In recent years, caves full of bats have been dying from white nose syndrome. And entire hives of bees have been going down for reasons that are still unclear.

Most of the time, when many animals die at once, they do it far from our daily lives in fields, caves, forests or national wildlife refuges, where animals often live at unusually high concentrations, and diseases can spread rapidly.
Flying from the sky

In Beebe, on the other hand, thousands of blackbirds had settled for the night in trees near people's homes. After a series of fireworks blasts went off on New Year's Eve, the birds were startled off their roosts. Because blackbirds can't see at night, they ended up flying all over the place, mostly downward.

One bird made it into a house, when the homeowner opened the door to see what was causing the racket. Most landed on roofs and the ground, making loud clunking noises as they shattered themselves to death. Necropsies revealed internal haemorrhaging, with no sign of pesticides.

"They weren't falling as dead carcasses," Meese said. "They didn't fall from the sky. They flew from the sky. That makes it less weird."

Another unique aspect of the Beebe incident was that noise played a role, says Paul Slota, a spokesperson for the USGS National Wildlife Health Center in Madison, Wisc. While the Swedish birds also died after a fireworks event, he said, mass die-offs are more likely connected to disease or weather.

"I think people are very often surprised that this kind of phenomenon happens, that wildlife are susceptible to disease and that there are large outbreaks in the wild, because they often go unseen," says Slota, who points out that the USGS investigates reports like these every year.

"I think people should be aware that mortality events in wildlife are normal. They are a fact of life."

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