On May 25, 1999, about 0005 Alaska daylight time, a deHavilland DHC-6-300 airplane, N72GC, sustained substantial damage when it struck a bird during the downwind portion of the landing phase, about 2 miles southwest of the Anchorage International Airport, Anchorage, Alaska. The airplane was being operated as an instrument flight rules (IFR) scheduled flight under Title 14 CFR Part 121 when the accident occurred. The airplane was operated by ERA Aviation Inc., Anchorage, as Flight 4857. The two flight crew members and the two passengers were not injured. Night visual meteorological conditions prevailed. An IFR flight plan was filed. The flight originated at the Kenai Municipal Airport, Kenai, Alaska, about 2348. The Director of Operations for the operator reported the flight was cleared for a right downwind approach for runway 14 at Anchorage. The airplane was about mid-channel between Fire Island, and the approach end of runway 6R. About 1,800 feet msl, the flight crew briefly observed several birds. The crew then felt an impact. Upon landing, damage to the leading edge of the left wing was discovered. Small portions of a bird were found on the leading edge. During an interview with the flight crew on May 27, 1999, the crew said the size and type of bird could not be identified. Personnel from the U.S. Department of Agriculture, Wildlife Services Program, responded to the Anchorage International Airport and gathered forensic, biological material from the accident airplane's wing surface. The material was submitted to the Smithsonian National Museum of Natural History for identification. The examination identified the bird as a Common Eider. Examination of the damage to the wing revealed denting of the leading edge, about four feet inboard from the left wing tip, and wrinkling of the upper wing surface. An internal examination of the wing disclosed two broken nose ribs.