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.