Wondering what may have caused approximately 5,000 red-wing blackbirds to fall dead from the sky over Arkansas on New Year's Eve?

Not to mention bird falls in Lousiana, Kentucky and Italy?

The U.S. Geological Survey says the phenomena are due to neither the End of Days nor of the Mayan calendar.

Large wildlife die-off events are fairly common, though they should never be ignored, said USGS scientists Monday, adding that their preliminary tests showed that the bird deaths in Arkansas on New Year's Eve and those in Louisiana were caused by impact trauma.

The birds were probably wakened and startled by loud noises on New Year's Eve, which caused them to fly off wildly and into hard objects, like trees and houses, the scientists said.

The preliminary findings from the USGS National Wildlife Health Center's Arkansas bird analyses suggesting that the birds died from impact trauma squares with the findings of the Arkansas Game and Fish Commission.

USGS scientists performed necropsies-the animal version of an autopsy-on the birds and found internal hemorrhaging, while the pesticide tests they conducted were negative.

Results from further laboratory tests are expected to be completed in two or three weeks.

Although wildlife die-offs always pose a concern, they are not all that unusual, said Jonathan Sleeman, director of the USGS NWHC in Madison, Wis., which is completing its analyses of the Arkansas and Louisiana birds. It's important to study and understand what happened in order to determine if we can prevent mortality events from happening again. 

In 2010, the USGS NWHC documented eight die-off events of 1,000 or more birds. The causes: starvation, avian cholera, Newcastle disease and parasites, according to Sleeman.

Such records show that, while the causes of death may vary, events like the red-winged blackbird die-off in Beebe, AK, and the smaller one near Baton Rouge, LA, are more common than people may realize.

According to USGS NWHC records, there have been 188 mortality events across the country involving 1,000 birds or more during the past 10 years. In 2009, individual events included one in which 50,000 birds died from avian botulism in Utah; 20,000 from the same disease in Idaho; and 10,000 bird deaths in Washington from a harmful algal bloom. 

Mass mortality events occur in other animal populations as well, according to USGS. For example, prairie dog colonies in the West can be destroyed by sylvatic plague, which can then kill off the highly endangered black-footed ferret that preys on prairie dogs exclusively. The USGS NWHC is involved with developing vaccines, delivered through bait, which can immunize prairie dogs against plague.