The young cuckoos fly to the south about a month after its true parent, following the same route by their instinct.  As it is a summer breeder and the voice or calls of Cuckoo can be heard until its migration to the south. As this old rhyme tells:  In April I open my bill, In May I sing night and day, In June I change my tune, In July far far I fly, In August away I must. So that is why we can not hear its calls during autumn time.

Cuckoo’s breeding behavior is the strangest wonders of natural world

There are 54 species of Cuckoos on our planet, and four of them have been recorded in Mongolia, they are summer breeder.

The young cuckoos fly to the south about a month after its true parent, following the same route by their instinct.  As it is a summer breeder and the voice or calls of Cuckoo can be heard until its migration to the south. As this old rhyme tells:  In April I open my bill, In May I sing night and day, In June I change my tune, In July far far I fly, In August away I must. So that is why we can not hear its calls during autumn time.