It usually burns up on re-entry

July 2006, STS-121; "An analysis of the inspections revealed the presence of bird droppings on the leading edge of the right wing. The launch director has said he saw droppings in that location before launch. During the crew press conference on landing day the crew joked about the bird droppings indicating that they were still present on the orbiter though rather charred." (Wikipedia)

I just decided to post this under today's date, I don't remember what day it ran in the Charter Oak-Ute NEWSpaper back in the summer of '06.

That wasn't the only bird trouble this shuttle had- "STS-121 was the first launch during which NASA took special precautions to deter vultures, particularly the local turkey vultures, from flying over the area. Efforts included the swift removal of roadkill and other dead animals from areas around the launch site." (also Wikipedia)

"Sometimes you're the windshield
Sometimes you're the bug"
~Dire Straights