Why is there a black dot in the middle of pure white bird droppings (the
people on the jokelist aren't surprised I would answer something like
this)?

Well, the black dot is the actual fecal matter; all of that white stuff
is a bird's urine. A bird's fecal matter and urine basically combine
together and are expelled at the same time out of the same orifice. The
fecal matter rests in the middle of bird droppings becuase the urine's
consistency is sticky and sticks to that other stuff.