A sparrow is small enough to get amongst the rudder pedals, into the avionics, and everywhere in between. Plenty of cooling ducts, spaces, and the less than happy thought of a frightened, crapping bird flying around the flight deck just as you get airborne... then an engine fails.... distracted, and it gets messy... the lawyers are going to get medieval on you.
Not even a debate. Find the bird or we don't go flying.