The saying goes that "all birds have their singing hour, but the cuckoo never stops." Other birds grow tired and rest, but the cuckoo doesn't sleep.
說來恐怕還和杜鵑真實的「生態」有關。所謂百鳥有啼時，子規聲不歇，人家叫累了，就休息，杜鵑卻不眠不休，晝夜不止。