On August 27, 2015, about 1110 central daylight time, a Piper PA-23-250, N900AM, sustained minor damage when it was struck by an unknown object while in cruise flight at 2,500 ft above mean sea level (1,920 ft above ground level) about 4 nm north of the Joliet Regional Airport (JOT), Joliet, Illinois. The commercial pilot was not injured. The airplane was registered to and operated by Aerial Imaging Inc. under the provisions of the 14 Code of Federal Regulations Part 91 as a positioning flight. Visual meteorological conditions prevailed at the time of the incident, and no flight plan was filed. The airplane departed the Aurora Municipal Airport (AAR), Aurora, Illinois, about 1100 and landed without incident at the South Bend International Airport (SBN), South Bend, Indiana. The pilot reported that he heard a "bang" and felt a jolt in the flight controls while he was completing the cruise checklist. He did not see any objects before the noise, and he did not see any damage to the airplane. The engines were operating normally, the flight controls felt normal, the landing gear indicated up and locked, and the cargo did not appear to have shifted. He continued the flight to SBN. No abnormal flight characteristics were noted during the flight. After landing, damage to the leading edge of the left horizontal stabilizer was observed. The damage included about four dents spanning approximately 13 inches from left to right with the inboard dent piercing the aluminum skin. The damage also included multiple scratches and cuts in the deicing boot that overlies the leading edge of the left horizontal stabilizer. The damaged section of the deicing boot was removed from the leading edge of the left horizontal stabilizer and shipped to the National Transportation Safety Board's Materials Laboratory for examination. The NTSB examination using an ultraviolet light indicated that insects impacted the deicing boot, but there was no widespread area that fluoresced under exposure to ultraviolet light consistent with an exposure to larger volumes of biological materials. The tears in the deicing boot revealed exhibited features consistent with failure from overstress. The deicing boot was subsequently examined at the Feather Identification Laboratory, Division of Birds, Smithsonian Institution National Museum of Natural History. Several feather barbs were removed from the tears in the boot during the microscopic examinations that were consistent with remnants from the Columbidae bird family (doves and pigeons).