I have found from experience that birthdays are automatically entered in Birthday calendar from Contacts. But if you delete the contact - BD stays in calendar. If you import same contact again you will have duplicate birthday entries for that contact. I have managed to solve that the "ugly" way: - deleted Birthdays calendar - exported all my contacts to file - reimported all contacts - restarted the device so that Birthdays calendar is created again. If you know what you are doing it takes 5 minutes with reboot time.