A kenning phrase consists of two words combined, usually by means of a hyphen, that function as a metaphorical description. Kennings are a tradition from Anglo-Saxon poetry. For instance, in the famous epic poem "Beowulf", the compound expression "battle-sweat" is used to refer to blood. Battle-sweat is thus a kenning, a phrase that metaphorically describes blood.
In the sentence we are studying here, we have a compound expression in "stoic-songbird." It is the combination of two words used to metaphorically describe a musical instrument. We could substitute it for the word "piano", for example. As we can see, using kennings helps embellish descriptions - stoic-songbird sounds more melodious than the word piano itself.
