i Register
In some senses, songbird is marked as idiomatic. Watch for register when choosing this word.
noun
A bird having a melodious song or call.
Sweet little lady sings like a songbird / And she sings the opera like you ain't never heard
Any bird in the passerine suborder Passeri, which are distinguished by a more sophisticated vocal apparatus.
A person, especially a woman, with a melodious voice.