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
Cuckoos do not have the instinct to build nests. In order to reproduce their offspring, they can only lay their eggs in the nest of 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