Hail stones?
Maybe. But I would think hailstones big enough to kill a bird would leave some other evidence.
And I would think the birds themselves would try to take cover. But then they are birds, that means bird-brains, so who knows?
Sounds like whatever it was, it was a very sudden and short event, and the affected birds basically didn’t have a chance.
I’m almost in the DOD weapons testing crowd here, but not yet...