“#3, we don’t hear about massive numbers of birds falling out of the sky on the 4th of July.”
Very good point.
That's not really much of a mystery- birds flock together in large numbers in winter both to move south and to keep warm; these flocks disperse to breed in spring and summer and wouldn't be affected by July 4 fireworks in any noticeable quantity. Without the flock, single or pairs of breeding birds may get just as startled but would not be in danger of ramming into other birds in midflight as they would in winter.
The particular species of birds in question are known for massive flocks numbering in the millions during wintertime, so thick they baken trees with otherwise bare branches. Given that in at least one of the locations [if not both] the birds were killed along city streets lined with trees where fireworks would be thick at that time and where the rough and cold weather would have caused the flocks to tighten up even more, birds packing together for warmth, it would be surprising not to find a number of dead or injured birds from small flocks but these species travel in huge focks and the casualties would naturally be significant.
Bald eagles up along the Mississippi often dive on flocks of snow geese that blanket the floodplains- not in the hope of catching any on the wing, but in the hope of causing those on the ground to take flight in fear and confusion. In their rush to get airborne the geese collide with one another in numbers and the eagles find it very easy pickings to go after these disabled birds. Coyotes and foxes clean up after he eagles leave. And these eagle attacks are in broad daylight when the geese can see perfectly well to maneuver. In darkness, with night vision wiped out by random flashes of intense light, the flockss of small birds roosting in street trees would panic all the more.