Explanation?: Most birds constantly eat grit to enable their crop to grind up food anyway. Did the macaws that just so happened to eat this clay have a survival advantage over others?
Perhaps harder to explain:
A parasitic wasp that must sting its host(a cockroach) in a precise region of its brain in order to get the right results.
There’s no room for error here and successful reproduction is at stake. What might have been the intermediary steps that led to this destination?