Which poetic device is primarily used in 'But words, like summer birds, depart?'

What does the poet suggest about the nature of words in the line 'But words, when summoned to the test, Oft satisfy the least!'?

In the poem, what metaphor is used to describe words?

What does the phrase 'Like plants that make a gaudy show' imply about words?

What does the poet imply when saying 'the world might hold a feast'?