Birds are very sensitive to electrostatic charges. A simple thunder cloud will irritate a bird severley. This is why birds fly low to the ground just before a storm, to avoid the very uncomfortable charges in the air.
In Arkansas there is one of the Worlds Largest Quartz deposits underground. Quartz is piezoelectric which means a charge developes over the surface of the quartz when pressure is applied to the crystal. The charge is millions of volts but it happens so fast its nearly harmless to people on a small scale. This is how some lighters work. A hammer hits a quartz device releasing a spark.
Anyways, the recent earthquake has placed new pressures on the quartz deposits in Arkansas. This results in a massive discharge of billions of volts of electricity. Think how a small piece of quartz the size of a pea can light a lighter. Now think about millions of Tons of quartz discharging under the extreme pressures the Earth can push.
At some point, the charges below created a brief surface charge, positive charge, during a discharge event. The electrostatic charge is immense and knocks the birds out cold in flight. The birds are still alive though, just knocked unconscious. The birds that were flying high were the ones killed upon impact. Blunt force trauma. The birds that were in the trees survived. Some birds like to fly high, others fly low. This explains why it was species specific as the birds that flew high were the ones to die.
Even American Indians knew the area in Arkansas to have very spiritual properties. Unknown but due to the massive quartz deposits below.
Now, there is a huge amount of pressure shifting below Arkansas. It is not uncommon to find mass animal death just before a large earthquake.....And the New Madrid Fault is due, over due infact.
I highly suggest anyone near the New Madrid Fault to store water and non perishables for a few weeks. The standard prep kit.