This sentence means that at dusk, the birds in the southern mountainous areas return to their nests to rest after being tired from flying.
"Angelica Angelica": means "returning to nest" or "going home". The two "Angelica" represent the sound or movement of birds returning to their nests. "Sunset" means that the sun is about to set and the sky is getting dark. "Nanshan": the mountain in the south, indicating the location in the poem. "Bird tired to fly": refers to a tired bird flying.