Almost nothing happens in this poem, and that's the point. The speaker is outside on a clear, freezing morning, paying attention to sounds rather than sights. First comes the small, sharp noise of a squirrel gnawing through a walnut shell, so quiet it can only be heard because everything else is still. Then a Carolina wren starts up, and the speaker compares its song to an engine: a "happiness motor" that turns over once, twice, then catches and runs.
That comparison is the heart of the poem. It's funny and warm, and it makes the bird sound like something cheerful and a little stubborn, slow to start but sure to get going. The speaker seems content, even delighted, though they never say so directly. The mood is calm, alert, and gently playful. The cold makes the morning feel crisp and clear, and the short lines mimic the quick, small sounds they describe. It's a snapshot of simply being present and listening.