"To Brooklyn Bridge" is a tribute to one of the world's most famous bridges, but it takes a few stanzas to introduce its main subject. The poem begins, instead, by describing the flight of a seagull near the bridge. By delaying the introduction of the bridge itself—the entrance of its main character, so to speak—the poem creates a sense of drama and expectation.
The description of the seagull is highly musical, full of short /i/ assonance and consonance:
How many dawns, chill from his rippling rest
The seagull's wings shall dip and pivot him,
Shedding white rings of tumult, [...]
The echoing /i/, /ip,/ and /ings/ sounds seem to ripple throughout the stanza, much like the "rippling" water around the "rest[ing]" bird. (Gulls often sleep on or near open water; this particular gull has been resting on the "bay waters" of New York Harbor.) The repetition also creates a sense of energy and momentum as the bird takes flight. Once aloft in the cold dawn air, the gull dives and wheels—"dip[s] and pivot[s]"—by adjusting his "chill[y]" wings. In the process, he seems to trace "white rings" in the air, and he "Shed[s]" white feathers due to the turbulence ("tumult") of his flight.
As he soars, according to the poet, the gull "build[s] high / Over the chained bay waters Liberty." This odd phrase mashes together several ideas into one dense image. Mainly, it suggests that the gull's flight creates, or "build[s]," an image of liberation that contrasts with the confined (metaphorically, "chained") waters of the "bay." Indeed, the bird seems to shed not only his feathers but any ties to the earth below. But the capitalization of "Liberty" also evokes another familiar sight in New York Harbor: the Statue of Liberty. The poet isn't claiming that the gull literally builds this statue, of course; rather, the gull's rising flight seems to create a comparable, figurative monument to freedom. It might even draw the viewer's eyes toward Lady Liberty. Moreover, the seagull does this repeatedly, morning after morning: "How many dawns [this happens]," the poet marvels, opening the poem in a tone of wonder.
If the poet seems to be squeezing a lot into these first few lines, that's because he's doing it on purpose! Hart Crane developed a style that flowed rapidly from image to image, metaphor to metaphor, in a fluid, intuitive, associative fashion. He called this technique his "logic of metaphor," and he used it to capture the dynamic, fast-paced modern world—including the busy cityscape this poem portrays.