Les oiseaux lents

They were called slow birds because they flew through the air - at a pace of three feet per minute. Slow birds always kept the same height above the ground and they would bank to avoid substantial buildings, but frailer obstructions they would push on through. No one could keep track of a single slow bird. After each one appeared it would fly onward slowly for any length of time between an hour and a day. And vanish again. To reappear somewhere else unpredictably. But how? And why?