Hi, i found a way how to do it but still it’s not automatic updated calendar.

1. Select Settings (Calendar settings in Google Calendar).
2. Go to the Calendars tab.
3. Follow the Export calendar link under My Calendars (You will get Birthday calendar in ics format)
   Next steps are optional
4. Create new calendar
5. Select Settings (Calendar settings in Google Calendar).
6. Go to the Calendars tab.
7. Follow the Import calendar link under My Calendars
8. Select your new calendar
9. Now you can use your public or private adress for modules

I have same problem, too. Any idea of solution?