As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER STARTING APU, A BURNING ODOR CAME FROM APU. WENT OUTSIDE AND FOUND FLAMES COMING FROM EXHAUST ALONG WITH DEBRIS FROM A BIRDS NEST. SOME OF THE STICKS AND GRASS WENT INTO THE INTAKE. SHUTDOWN APU AND DISCHARGED A PORTABLE FIRE EXTINGUISHER. CLEANED OUT APU INLET SCREEN AND TAIL EXHAUST TUBE. INSPECTED APU, NO DEFECTS NOTED. PERFORMED OPERATIONAL CHECK OF THE APU. OPS CHECKED GOOD IAW MM 49-10-00.