The speaker is at the end of a blustery day, listening to starlings settle down for the night. The wind has dropped, and in the new stillness the sleeping birds make odd little noises that sound like pieces of car alarms. That small, slightly comic comparison pulls the natural and the human-made world together, and it prompts a memory.
The speaker recalls the moment the flock burst up from the forest floor, flashing with iridescent color and pouring through the branches. The poem compares them to a rush-hour crowd, and then to a dark road unrolling across the sky. The traffic image echoes the "fast traffic of clouds" earlier in the poem, so the day's movement and the birds' movement seem to belong together.
The mood is quiet, observant, and a little wondering. Calm in the present contrasts with the energy of the remembered flight, and the speaker seems fascinated rather than troubled by how much the birds resemble our own busy world.