Three short lines, and a quiet question that doesn't need an answer. The speaker is outside or at a window as the light fades, and a small curiosity surfaces: what color are the birds' heads now? In daylight the answer would be obvious, a red cap or a green sheen or a plain brown. Once the sun is down, that detail disappears, even though the birds are presumably still there somewhere.
What the speaker feels is gentle, idle wonder rather than worry. There's no urgency in the question. It's the kind of thing you think about when your attention has softened at the end of the day.
The mood is hushed and a little dreamy. The poem points to how darkness hides things we know are real, and how the mind keeps picturing them anyway. By stopping so abruptly, it leaves us in that dim space with the speaker, still wondering, noticing how much of the world goes unseen when the light is gone.