" ... swamp gas mixed with the reflections off the feathers of migratory birds ... "
Actually, "I hate to tell ya," that actually is true.
The "secret" is ions in the atmosphere and beyond; it's simple, really. Ions react to electromagnetic influence (and gravity); the result "in the randomness of things," is that people really do see something on occasion.
In the current Era of Super-Electromagnetism, if you will, there is ever-more "junk" being emitted and thence affecting ions in the heavens.
A "holographic effect" is there, serendipitously (sp?).
Yet because of one's perspective, some parties will see what others, "over yonder" will not.