The term, in Mandarin slang, for "bird shit" is pronounced tian1 shi3. This, in and of itself is nothing particularly interesting, but within the etymology behind it exists a bit of Chinese humour, for it is a homophone of the Mandarin word for "angel" ("天使", or "heavenly messenger"). Pronounced the same, and out of context -- or even within context, but in a moment of amusement -- the two terms are indistinguishable.
With this said, the humour is fairly straightforward, and immediately obvious -- it is "shit from heaven" -- a perhaps most apt description to anyone who has driven or been caught walking beneath a flock of seagulls.