A mysterious rain of thousands of dead birds darkened New Year's Eve in Arkansas, and this week similar reports streamed in from Louisiana, Sweden, and elsewhere. (See pictures of the Arkansas bird die-off.)
But the in-air bird deaths aren't due to some apocalyptic plague or insidious experiment—they happen all the time, scientists say. The recent buzz, it seems, was mainly hatched by media hype.
At any given time there are "at least ten billion birds in North America ... and there could be as much as 20 billion—and almost half die each year due to natural causes," said ornithologist Greg Butcher, director of bird conservation for the National Audubon Society in Washington, D.C.
But what causes dead birds to fall from the sky en masse? The Arkansas case points to two common culprits: loud noises and crashes.
Beginning at roughly 11:30 p.m. on New Year's Eve Arkansas wildlife officers started hearing reports of birds falling from the sky in a square-mile area of the city of Beebe. Officials estimate that up to 5,000 red-winged blackbirds, European starlings, common grackles, and brown-headed cowbirds fell before midnight.
Results from preliminary testing released Wednesday by the National Wildlife Health Center in Madison, Wisconsin, show the birds died from blunt-force trauma, supporting preliminary findings released by the Arkansas Livestock and Poultry Commission on Monday.
"They collided with cars, trees, buildings, and other stationary objects," said ornithologist Karen Rowe of the Arkansas Game and Fish Commission.
"Right before they began to fall, it appears that really loud booms from professional-grade fireworks—10 to 12 of them, a few seconds apart—were reported in the general vicinity of a roost of the birds, flushing them out," Rowe said.
"There were other, legal fireworks set off at the same time that might have then forced the birds to fly lower than they normally do, below treetop level, and [these] birds have very poor night vision and do not typically fly at night."
The dead birds found in Arkansas are of species that normally congregate in large groups in fall or winter. "The record I've heard is 23 million birds in one roost," Audubon's Butcher said.
"In that context, 5,000 birds dying is a fairly small amount."
A Towering Problem for Birds
Birds often hit objects in flight, especially "tall buildings in cities, or cell phone towers, or wind turbines, or power lines," Butcher said.
"The structures that seem to cause the most deaths are very tall and constantly lit," he said. "On foggy nights, birds that should probably normally be paying attention to the stars get disoriented, and circle around the structures until they collapse" and fall.
Collisions with power lines seem to have killed roughly 500 blackbirds and starlings in Louisiana on Tuesday. The 50 to 100 jackdaws found on a street in Sweden that same day showed no signs of disease and also apparently died from blunt-force trauma, according to the Swedish National Veterinary Institute.
Wind, snow, hail, lightning, and other challenges posed by weather can easily kill flying birds too.
For example, "last year a couple of hundred pelicans washed up by the Oregon-Washington border," Butcher said. "A cold front had unexpectedly moved in, and they faced icing on their wings and bodies."
(Also see "Bird Color Mysteries Explained.")
Bird-Death Hype Detracts From True Crises?
Of course, death doesn't just stalk birds from above. For instance, "waterfowl get botulism—and salmonella and avian pox can spread at bird feeders," Butcher said.
No matter how it arrives, death appears to be very much a fact of life for birds. "Young birds that hatch in the spring have an approximately 75 percent chance of not reaching their first birthdays," the Arkansas Game and Fish Commission's Rowe said.
"To biologists, these deaths are normal occurrences.
"I wish I could take all this energy and attention on these deaths and direct them toward true crises in wildlife biology, to things like the white-nose syndrome in bats," Rowe added.
She does, though, see a silver lining in the sky-is-falling coverage this week.
"I hope we can raise public awareness of what impact man-made structures can have on other species. How many migratory warblers do you want to kill just to get better cell phone reception?"