When your brain fudges the results of what you're seeing because of some hidden processing heuristic. If the bird were actually just colored black, then what we would see would be accurate. But its presented as if we should percieve continuous movement when we really shouldn't.
I’m just saying upon initial thought that it seemed too simple to be considered an “illusion” because it doesn’t rely on some mental trickery, just blindness. But I’m willing to accept that optical illusions may all rely on some form of blindness. Glad to spawn the conversation!