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The mystery of the deaths of thousands of blackbirds in Arkansas this month has been solved.

They died of blunt-force trauma, according to the National Wildlife Health Center in Madison. Samples of the blackbirds - and other blackbirds from a separate mass die-off about the same time in Louisiana - were brought to the little-known laboratory on Madison's west side for necropsies.

"They died of impact force to their bodies," said Scott Wright, chief of disease investigations at the center.

He said the birds clearly showed signs of bruises.

The Arkansas Game and Fish Commission said tests for evidence of pesticide poisoning were negative.

At least 3,000 red-winged blackbirds died on New Year's Eve near Beebe, Ark. Fireworks probably sent them flying from their roost sites.

The Arkansas Game and Fish Commission said the agency began receiving reports of blackbirds falling from the sky about 11:30 p.m. Dec. 31 in a 1-square-mile area of Beebe.

The mass kill spawned speculation that their demise was caused by a chemical pollutant - a fear heightened by the arrival in Beebe of workers combing neighborhoods in hazmat suits. As the story spread, other theories ranged from biblical apocalypse to proof of the presence of UFOs.

"It's believed that the noise startled them - they are poor night fliers - and they were in close proximity to neighborhoods, and they flew into homes and cars (and other objects)," Wright said.

A separate incident near Baton Rouge, La., involving about 450 blackbirds is still under investigation by the center, Wright said.

"I think the two events are a coincidence," Wright said.

Also unrelated, he said, is the death of more than 80,000 freshwater drum, and a few yellow bass, white bass and sauger in the Arkansas River reported on Jan. 3.

Deaths not uncommon

Wright said large-scale deaths of birds and other species are not uncommon.

Storms, for instances, wreak havoc on bird populations. Birds are also vulnerable to chemical pollution and biological poisoning from natural toxins.

For the blackbird species alone, there have been 16 incidents in which 1,000 or more birds have died in single events over the past five years, he said.

The lab analyzes 300 to 500 large die-off events each year. The deaths involve all wildlife - deaths in 2010 are as varied as the demise of 4,500 bats from a fungal infection known as white-nose syndrome in Bucks County, Pa., to the death of 150 raccoons, striped skunks, coyotes and red fox in Los Angeles County.

The lab's website shows nine separate mortality events since December alone. Eight of those events involved bird species and one involved the gunshot deaths of dozens of Brazilian free-tailed bats near Pima, Ariz.

The National Wildlife Health Center also played a key role in the study of chronic wasting disease in Wisconsin's white-tailed deer population after an outbreak in February 2002.

"We don't know what we are going to get tomorrow, or the next day, but it will be something," Wright said.