Rio Douro, a 205-ton dragger type fishing vessel, was built in 1936 at Thomaston, Maine. The Navy purchased her at the end of October 1940, renamed her Mockingbird (AMc-28) and placed her in service in June 1941. The minesweeper was soon sent to tidewater Virginia, and served in that region for the next two and a half years. Mockingbird was placed out of service in February 1944. Stricken from the Navy list in late July of that year, she was transferred to the War Shipping Administration for disposal in July 1945.