In these two poems, flowers and birds should be cheerful and vivid scenes, but in the poet Du Fu's pen, they have become a symbol of sadness and desolation. This is because the poet Du Fu, in the face of pain and helplessness, feels that he is in the same boat with flowers and birds, and his heart is full of sadness. This technique of writing sadness with joy is called contrast.
In contrast, the poet highlights his inner sadness and desolation by describing the beauty and joy of flowers and birds. This technique can make readers feel the poet's inner feelings more deeply and make poetry more vivid and touching.