In an effort to broaden my post categories....

I was talking with a very wise friend of mine a while ago and we got on the subject of why bird poop is white.  He started off describing that humans have one of the most inefficient bodily waste systems.  We have seperate functions for excreting liquid and solid waste... As a result, our kidneys excrete nitrogenous wastes as urea which ends up getting dissolved in urine... 

On the other hand birds are unable to urinate so their kidneys excrete nitrogenous wastes in the form of uric acid.  Uric acid has a low solubility with water, so it comes out as a white paste.  Combine that white paste with some intestinal output and you get white bird poop sprinkled with little black bits.