This could be caused by an event such as a coming earthquake.
"...the birds had suffered internal trauma. That could have happened if a single flock had suddenly got caught in a violent and unusual storm. Or, it has been speculated, a local fireworks display could have startled the birds so badly that they were unable to prevent themselves from flying into trees, pylons and houses.
Bird experts stressed no one should be worried. "Mass bird die-offs can be caused by starvation, storms, disease, pesticide, collision with man-made structures or human disturbance ... Initial findings indicate that these are isolated incidents that were probably caused by disturbance and disorientation," Greg Butcher, director of bird conservation at the Audubon Society, said."
Of course, there are the folks who believe it's a sign from God, that He's had enough and he's coming down to kick everyone's ass.
YOU MAKE THE CALL!