Birds are the obvious subject here, but the poem reads just as much like a reflection on change in the seasons and in the climate. The opening lines suggest that the north no longer draws the birds as it once did, and the weather details back this up: mist hangs around too long, storms smell tropical, and ravens seem to crowd out the sun. Something is off, and the birds sense it.
Then the poem turns. The birds travel light, guided by a magnetic field, and the speaker seems to admire that simplicity. What they arrive at is home, which has grown "beyond elaboration," too rich to explain. The imagery becomes warm and sensuous: brightly feathered mountains, fermenting berries, a slow burn, and finally a night sky speckled like an egg.
The mood moves from unease to comfort, even wonder. The closing image of the sky as an egg suggests shelter and a new beginning, as though leaving is also a kind of return.