Three short lines, and the last one lands like a punch. An owl calls in the garden, and the speaker goes outside and vomits. The poem never says why, and that gap is what makes it so unsettling.
An owl's cry is usually a gentle night sound, maybe even a pleasant one. Here it triggers a physical reaction that seems far too big for the cause. The calm, flat way the speaker reports it, with no punctuation and no explanation, makes the reaction feel involuntary, as if the body responded before the mind could weigh in. Judging by the abrupt shift, the call may stir up fear, dread, or some memory or superstition tied to owls, though the poem leaves that open.
The mood is tense and eerie. Because we only get the sound and the sickness, we fill in the rest ourselves, and that uncertainty is where the poem gets its chill.