According to the pilot, during takeoff, when the aerial application airplane was about 15 feet above ground level, he noticed a decrease in engine performance, verified that the engine controls were full forward and turned the fuel boost pump ON. He dropped the hopper load, however the airplane performance continued to decrease. The pilot elected to land the airplane straight-ahead and the airplane impacted trees prior to coming to rest inverted in a bog. Postaccident examination of the airplane revealed substantial damage to the fuselage and wings. Furthermore, after the airplane was removed from the bog, the Federal Aviation Administration inspector who responded to the accident found feathers associated with a Canadian goose in the vicinity of the engine and damaged propeller. According to the pilot there were no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.