The term 'sweet-singing bird' can be summarized by two crossword solutions: THRUSH (6 letters) and vireo (5 letters). A THRUSH is a type of small, plump-bodied songbird with a brown back and speckled breast. They are known for their melodic songs and are found in woodlands and gardens throughout the world. A vireo is a small, greenish-olive songbird with a white or yellowish belly. They are found in North and South America and are known for their sweet, warbling songs.