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
All kinds of forest pests are the delicacy of the cuckoos, but the only food of the willow warblers is the pine moth.
 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