The first thing people need to understand is why these mass collision events occur. It takes three major factors to occur simultaneously, a perfect storm, to cause a mass collision event. During migration, nocturnal migrants usually choose clear nights with favorable winds. The favorable winds the birds look for sometimes occur at very high altitudes (10,000+ ft), but in general it is safe to say that most are flying thousands of feet above the landscape for most of their journey. On these clear nights nocturnal migrants are far above and safe from the effects of city lights. However, when nocturnal migrants fly into less-than-favorable weather conditions like low, heavy clouds, fog, and precipitation, which can force them to fly lower, their troubles begin. If you have tens of thousands of migrants already on the move, and they run into these unfavorable conditions near or over a city with bright lights, this is when and where mass collisions are possible. Imagine driving along a highway on a clear day and suddenly being hit by either a heavy downpour or a heavy snow squall. You can go from being able to see fine to being totally blind in seconds. Even a better example of what the birds can experience is driving along on a clear night and running into a thick fog bank with blinding reflections of white. So, for a mass collision event to occur, three things must come together: 1) Large concentration of migrants, 2) Inclement weather, and 3) A bright city nearby. All three conditions at the same time is not so common, which is why even nationwide these events are not reported annually.
This is not to say that thousands of birds are not forced down more often, into the surrounding neighborhoods and suburban areas, only to face the uncountable windows at dawn, as well as outdoor cats and the multitude of other human-caused hazards. There, when a bird dies here and there, even if thousands die in a single morning, they go mostly unnoticed. Mass events in downtown areas occur where sidewalks don’t hide the dead and dying, hundreds of people witness this, and reporters document it. By contrast, the casualties that are spread out through large areas—with vegetation often hiding the bodies—are simply not counted.
The annual USA death toll due to bird-window collisions is between 365 million to one billion. Using the LOW estimate equals an average of one million a day. A peer-reviewed, scientific paper stated that 54% of window collisions occur at low-rise buildings like college and corporate campuses, 45% occur at residential homes, and only 1% occur at high-rise buildings.