Symbolism, Imagery, Allegory
Sometimes we hit the jackpot when reading poetry and the poet tells us how to interpret an image. This is the case with the sea-swift (that's a common term to you and me):
[…] she was the mind-
messenger, and her speed outdarted Memory.
She was the swift that he had seen in the cedars
in the foam of clouds, when she had shot across
the blue ridges of waves, to a god's orders,
and he, at the beck of her beak, watched the bird hum
the whipping Atlantic, and felt he was headed home. (XXIV.iii.131)
And if that's not enough for you, God himself descends into the book to give us a bit more, saying: "Is I send the sea-swift as a pilot,/the swift whose wings is the sign of my crucifixion" (XXV.i.134). Well, there you go.
Let's not be lazy about this, though. Just because one character (even if it is God) tells you to think one way about something doesn't mean that's the end of interpretation. When we first see the swift, she is part of Achille's observations. He glories in her beauty and thinks about his fondness for the sea—until things get creepy.
In the throes of sun-stroke, Achille realizes that perhaps the bird "was no swallow but the bait of the gods,/that she had seen the god's body torn from its hill" (XXIV.i.126). Yikes. He is understandably (and rightly) freaked out about the creature at this point. This isn't just a bird—it's an archetype.
The swift drags Achille on a journey that he doesn't necessarily want to take, even though it is crucial to his understanding of his personal identity. In essence, the sea-swift, in addition to all those lofty things we already know about her, unlocks Achille's ancestral past for him. And when she does this, she brings the two halves of his life together: his African roots and his St. Lucian present.
The swift, then, becomes a symbol of healing in the poem. Recall that it is a swift that carries the seed that will heal Philoctete all the way from Africa in its belly (XLVII.iii.238-239). It's also a swift that carries away the souls of the dead (XXXV.ii.179) and a swift that becomes a figure for Catherine Weldon's pen—which offers solace to our grieving narrator (XXXV.iii.181). Yup, this bird's on a healing spree.
Just as the swift heals the division in Achille's identity, she also helps the poet reconcile some big ideological difficulties, and solve some narrative problems:
I followed a sea-swift to both sides of this text;
her hyphen stitched its seam, like the interlocking
basins of a globe in which one half fits the next
into an equator, both shores neatly clicking
into a globe […]
Her wing-beat carries these islands to Africa,
she sewed the Atlantic rift with a needle's line,
the rift in the soul. (LXIII.iii.319)
The swift allows Walcott to talk about two different worlds simultaneously and helps him bring together two halves of the Caribbean soul to create a coherent and comprehensive story of his people. She's a pretty amazing character, for a bird.