Why are birds falling from the sky?

As I was watching an episode of one of my interesting channels National Geographic, they brought up something about an abnormal event of a mysterious rain of thousands of dead birds darkened New Year's Eve in Arkansas, they found out that bird-deaths aren't due to some apocalyptic plague or insidious experiment that happens all the time. Here are some pictures below:-

At any given time there are "at least ten billion birds in North America ... and there could be as much as 20 billion—and almost half die each year due to natural causes including Wind, Snow, hail, lightning and other challenges that pose threat to birds, and also what causes birds to fall from the sky as the Arkansas case points into two common culprits; loud noises and crashes. It also seems that birds die-off from blunt-force trauma as they are also collided with cars, trees, buildings and other stationary objects, they also seem to collide with power lines, strangely, these power lines have killed about 500 black birds and starlings in Louisiana. The legal fireworks also pose threat to these birds because some birds congregate at night unwillingly due to sudden effect on their habitat, so when these birds congregate at these times, sometimes they are forced to fly lower than they normally do to avoid collision with treetops as they have a very poor night vision, so as they fly in a low height they are may be affected by the fireworks. The dead birds found in Arkansas are of species that normally congregate in large groups during Fall or winter, well this explains why they fall in large numbers.