Investigators at the National Wildlife Health Center in Madison, Wis., who examined a sample of the dead birds discovered they were killed by blunt force trauma, the Milwaukee Journal Sentinel reported. Scott Wright, head of disease investigations, said the birds' bodies were bruised and they had been bleeding.
Wright said it is probably no coincidence the mass die-off occurred on New Year's Eve. He said they were most likely startled while roosting by fireworks.
"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," Wright said.
The center is also examining samples of 450 blackbirds found dead near Baton Rouge, La. Wright said mass die-offs of birds happen more frequently than most people realize, and the two events, although they occurred in neighboring states a few days apart are likely unrelated.
In the past five years, there have been 16 die-offs of 1,000 birds or more just for blackbirds, Wright said.