Mike,
Usually this is due to an updraft lofting the birds into the upper atmosphere.
A collision between warm and cold fronts over Arkansas is the most probable explanation.
The birds get hypoxic, then fall out of the air.
Another possibility is blast from a big lightning strike - death by thunderclap(!)
but there seem to be too many birds for that.
A stream of intensely cold air seems unlikely - most birds are fairly robust and should be able to fly out of the cold zone. Sudden unconsciousness from a blast of cold is possible, but it would have to be really really cold!
If the birds were all cooked or frozen, *then* we're dealing with something truly weird.
Fish kills are generally due to changes in water temperature (warming reduces oxygen content), oxygen content or pH.
Algal blooming and fish kills from algal toxin release is being seen more often around the world secondary to fertiliser run-off.
Sewage contamination will reduce water oxygen levels.
Toxic industrial contamination or landfill runoff is hopefully becoming less of a problem.
Rob
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