Something stunned the birds in flight then they died of smacking into the ground?
I don’t think they died smacking into the ground, or any other hard thing for that matter.
Because all there was (AFAIK) were dead birds. There were no birds with broken wings or legs or broken necks.
There (once again AFAIK) no reports of piles of feathers or trees or buildings with incidental feathers attached indicating high speed collisions killed them
They basically were dead when they hit the ground.