The term 'Feather Source' can be broken down into two crossword solutions: EMU (3 letters) and EIDER (5 letters). An EMU is a large, flightless bird native to Australia that is the second-largest living bird by height. Its feathers are used for a variety of purposes, including clothing and decorations. An EIDER is a large sea duck that is found in the Northern Hemisphere. Its feathers are used for insulation in clothing and bedding, as well as for decorative purposes.