What does the phrase summer sentinel meaning a cuckoo is an example of?

1. define "sentinel" in a dictionary 2. look up "cuckoo" in Wikipedia 3. Think about what a sentinel does, and how a cuckoo is like that, with respect to summer. 4. Examine how that metaphor works in the context of The Seafarer poem. Where does it appear in the poem, and why? ---- In Anglo saxon poetry (and several other medieval poetries) there was a special technique by which instead of directly naming a thing, the poet would describe it and leave the listener (or reader) to work it out.

The ocean is 'the whales' highway';

snow is 'the sifting from heaven's threshing floor';

a girl's hair is 'the golden hayrick over her ears'.

To call a cuckoo a 'summer sentinel' is an example of such a phrase. The technical term (in English) is a 'kenning'.