The wood thrush in T.S. Eliot's "Marina" shows up in two places in the poem; the beginning and the end. The poem is about the life of a seaman and the fact that wherever he goes, there are similarities in the landscapes. He's been at it so long that all of the places he visits, all of the "marinas" in which he docks, look the same. This is illustrated by his complete and utter knowledge of his boat as described here:
Bowsprit cracked with ice and paint cracked with heat.
I made this, I have forgotten
The rigging weak and the canvas rotten
Between one June and another September.
Made this unknowing, half conscious, unknown, my own.
The garboard strake leaks, the seams need caulking.
The boat looks the same to him because he's been on it for so long. In the same way, the marinas he visits look the same because he is used to them and he can't tell them apart anymore. The "woodthrush" (a bird that flies around at the marinas) illustrate the "sameness" of his journeys and his ports. Wherever he goes, there's a woodthrush to greet him. Here are the two stanzas:
What seas what shores what grey rocks and what islands
What water lapping the bow
And scent of pine and the woodthrush singing through the fog
What seas what shores what granite islands towards my timbers And woodthrush calling through the fog
Though he's in a different marina in each stanza, he sees the same images of fog, rock and woodthrush.