Locals in Beebe, Ark., were mystified when as many as 5,000 red-winged blackbirds dropped out of the sky Dec. 31.
Studies commissioned by the Arkansas Game and Fish Commission ruled out possible causes such as disease and poison.
The tests revealed hemorrhaging "consistent with blunt trauma," according to a Southeastern Cooperative Wildlife Disease Study. "In most instances, such traumatic injuries in wild birds are due to flying into stationary objects such as trees, houses, windows, power lines, towers, etc."
There has been a string of mass animal deaths so far this year, from dead fish in the Detroit River to hundreds of dead starlings in South Dakota.
Arkansas blackbirds have poor eyesight and don't normally fly at night. The AGFC said the birds were probably disturbed by "unusually loud noises" and flew lower than normal due to New Year's Eve fireworks. The rare night flight was even recorded on radar data.
"The first exodus occurred about 10:20 [p.m.] and contained approximately 6,000 to 7,000 birds per cubic kilometer," said Sidney Gauthreaux, professor emeritus at Clemson University. "At 11:21 p.m., another pulse of birds with a slightly smaller density left the roost."
A local said that the birds made the streets of Yankton, S.D., look like something out of an Alfred Hitchcock movie.
In Arkansas, the mystery of the red-winged blackbirds may be solved, but officials are still mystified as to what killed more than 80,000 freshwater drum in the Arkansas River last December.
So far, officials have ruled out bacteria, viruses and parasites as the culprit, according to the Arkansas Times.
"Unfortunately, we probably will never know exactly what killed these fish," said AGFC Assistant Chief of Fisheries Chris Racey.