Thousands of birds in flight killed at different locations sounds like some sort of experimental high-powered beam, likely intended for use as some sort of radar or communications device. They can't talk about top secret tech stuff in the paper and say "oops" so they say something silly like "fireworks dunnit."

A few years back a new type of Navy sonar was resulting in massive fish kills, which they also tried to cover up in a similar manner.

Doubt very much it is anything to worry about. Just the usual bungling.