A seagull lands on a traffic island to inspect a discarded chip covered in ketchup and mayonnaise. It sounds like a small, almost comic street scene, and the poem begins that way, with plain, flat description. But the speaker's attitude shifts quickly. "I have never liked gulls," she says, and from there the bird grows menacing.
The wings become "seizing arms," and the gull seems to loop around her head as though it might attack. The speaker imagines it taking "what it wanted": the chip, but also her "mind." That suggests a fear that is bigger than the situation, a sense of losing control. The bird's "poker-face" gives it a cold, unreadable calm, and the final line, "it circled for death," pushes the mood into real dread.
The effect is a slide from the everyday into unease. Something ordinary starts to feel like a warning, and the speaker's anxiety fills the scene.