The cuckoo cries for blood is an allusion. It is said that Du Yu, the monarch of the ancient Shu kingdom, was also called Wangdi. He was forced by his courtiers and fled to the mountains. After his death, he was worried and turned into a bird, called the cuckoo bird. He cried all day long, even his mouth bled and blood flowed to the flowers, which was the azalea. In ancient poetry, the cuckoo is a kind of sad bird, but whenever the heart is sad, it is often expressed by this bird, so whenever the cuckoo appears, it is to express a kind of sadness.
In fact, cuckoo is cuckoo, and because of its sound "cuckoo", like "Hu Bugui" (why not return), it has become a symbol of homesickness.
anyway, cuckoo is a sad bird, full of sadness.