Exams at the National Wildlife Health Center showed the mass die-offs in Arkansas were a result of impact trauma. No blunt force object but rather collisions. They found no evidence of poisonings or pesticides. Scientists are thinking the culprit was commercial-grade fireworks that went off. The Louisiana bird die-offs are still being investigated, but initial information shows those deaths may have been related to weather.
I visited the National Wildlife Health Center not too long ago. This little-known federal lab, which is run by the US Geological Survey, does innovative work investigating wildlife diseases, from white-nose syndrome in bats (it identified the fungus that causes the illness), to chronic wasting disease and the West Nile virus.