Symbolism, Imagery, Allegory
When he returns from his spirit-journey across the Atlantic, Achille and his mate spot a large, black frigate-bird in the air above the island. The men comment on the relationship between the white gulls and the black bird, which is quite the opposite from the racial interactions in their lives: "Them stupid gulls does fish/for him every morning. He himself don't catch none,/white slaves for a black king" (XXX.i.158). This is about as inverted a dynamic as possible in this book.
The two men comment on the idiocy of the situation, Achille even pointing out the injustice of the arrangement. And yet, there is something admirable in the bird, something beautiful and proud. This something brings Afolabe to Achille's mind, and he sees the name of his father written in the sky with the tips of the frigate-bird's wings.
So it is that the frigate-bird symbolizes a righting of the social order for Achille and his people—or at least a restoration of dignity that was lost through enslavement and poverty. The bird is a reminder of the power and pride that belongs to each human being.
It's the cry of the frigate-bird that brings the narrator back to his senses after he stupidly insults Omeros on their trip to the Underworld ("I haven't really read all of your works," he says (LVI.iii.283)) and helps him voice his true admiration for the great poet.
It's no coincidence, then, that the bird finds its way back into the text when the narrator makes his poetic voice heard in praise of the island: "My voice was going/under the strength of his voice, which carried so far/that a black frigate heard it, steadying its wing" (LVII.i.287). Just as the frigate-bird is connected to the dignity of Achille and his people, in its connection to the narrator's voice, it's connected to his dignity as well.
While the sea-swift stands for something more ethereal—even supernatural—the frigate-bird represents St. Lucia itself (be sure to read up on the sea-swift elsewhere in this section). As a reminder of a proud but lost past, and the sign of the natural majesty and beauty of the island, it acts as a kind of spiritual compass for the wanderers in this text.