Here's why:
* Metaphor: A figure of speech where a word or phrase is applied to an object or action to which it is not literally applicable.
* Literal Meaning: A cuckoo is a bird known for its distinctive call.
* Figurative Meaning: The cuckoo is being compared to a sentinel, a guard or watchman. This suggests that the cuckoo's call is a warning or signal, much like a sentinel's.
The poet uses this metaphor to evoke a sense of watchfulness and anticipation, reflecting the speaker's vigilant and solitary existence as a seafarer. The cuckoo's call becomes a symbol of the passing of time and the cyclical nature of life.