An extended metaphor is a type of comparison between two unlike objects and it moves throughout the poem or a long passage. It is when poets or authors use a single metaphor with various linked tenors, vehicles and grounds. It is also known as sustained metaphor or conceit.
In the following example, an extended metaphor moves through the entire text. First, the poet compares someone’s world to something very big and then gives a stark contrast of his world, which is very small and narrow like a bird in a nest narrowing his wings.