Poets have always been listening. The meanings they seek to convey in their poems often seem to lie half outside the words, in the rush of wind or water, in the thunder, in the cries of birds, as if poets were trying to translate into human language a poetry that preexists in the whole body of the world's sounds. Composers also listen. When they read poems, they listen both to the music of the words themselves, and to the music on the far side of the poems, the music that the poets themselves were attending to. So when Haydn sets a passage in The Seasons in which frogs appear, he sets the orchestra a-croaking. The philosopher Schopenhauer greatly deplored this tendency in Haydn, on the grounds that music should strive to align itself with the deep urgencies hidden in the heart of things, and not to imitate external phenomena. But it's futile to try to argue Haydn out of his ribbits, or Beethoven out of his cheep-cheeps in the song “Die Wachtel” (“The Quail”). In a poem about sound, the external sound is an irresistibly potent metaphor for the poem's meaning. Imagine a composer trying to set this passage from a poem from 1646 by Richard Crashaw, concerning a nightingale:

Only an extraordinary chaste composer could resist the temptation to embody in the music something of the dynamic of the whirlwind and the earthquake, the immense pressure trying to escape through the small orifice of the bird's throat. The meaning of the poem may not be exactly identical with tornado, tremor, or throb of song, but it would be difficult to convey the poem's meaning without some audible allusion to these things.