Reports of dead birds causing little concern in Michiana, WSBT, January 6, 2010:
Wendy Jones, head naturalist at Fernwood Botanical Garden and Nature Preserve, said visitors have come up with their own hypothesis of what happened down south, including "the birds hit each other after getting scared from noise (of the fireworks).
But Jones believes this theory is highly unlikely because birds are used to flying in flocks.
Her guess is that the birds got into some kind of poison, resulting in the same effect to all of them at the same time.
As far as the theory about the birds getting killed by a bolt of lightning, she believes that explanation isn’t very plausible either.
Second state has mass bird kill, SIFY News, January 6, 2011:
Dan Cristol, a biology professor and co-founder of the Institute for Integrative Bird Behavior Studies at the College of William&Mary... was skeptical of the fireworks theory, unless "somebody blew something into the roost, literally blowing the birds into the sky."