Specific birds:
Crane, the hermit's companion.
Phoenix, a symbol of nobility.
Chicken, a symbol of mediocrity.
Yuanyang, the symbol of husband and wife.
Magpie, good news.
Crow, undertaker.
Sparrows, finches, people who have no ambition, or people who are mean or have small ambitions.
Eagle, swan, ambitious.
Harrier, owl, fierce.
Owl, owl, vicious or violent person.
Goose, messenger.
This bird is a description of the scenery to contrast the feelings of the protagonist. Or happy, or sad, or sad, or lovesickness, depending on the specific poetry.