Because of the climate change in recent years, the cuckoos that used to live in the deep mountains of the Qinling Mountains in spring and summer gradually migrated to the Greater Khingan Mountains in the Northeast China  Just when the cuckoos were breeding in the the Greater Khingan Mountains, the number of willow warblers was greatly reduced
Both the willow warblers and the cuckoos feed on forest pests, but the cuckoos do not eat pine moth which is the main food of the willow warblers.
 Due to the different food of the cuckoos and willow warblers, the large decrease of the willow warblers has nothing to do with the large increase of the cuckoos