Straw contaminated with the H5N1 strain of bird flu was the likely source of an outbreak of the disease at a poultry farm in southern Germany that resulted in 160,000 birds being slaughtered, an official said Monday. Ottmar Fick, the chief veterinarian in the Erlangen district of northern Bavaria, said it remained unclear how the straw, which was stored on the farm, became infected, although wild birds were a possible source.