Mystery solved.

It wasn't poison, or lightning, or the end of the world. 

According to USA today: -

It was someone shooting off professional grade fireworks in a residential district, scaring the night-blind birds out of their roost into a 25-mph flight that ran them into houses, signs and even the ground, says Karen Rowe, Arkansas Game and Fish Commission ornithologist.

Rowe confirmed the cause of death after getting back results from extensive necropsies done on the animals and examining the pattern the birds were found in. Both were consistent with their theory that a startled flock flew up, then quickly back down to find safety and, unable to see at night, hit the ground and other objects at full speed.