Google contacts birthday's in Thunderbird/Sunbird

For some reason, there is no ical address to the contact birthdays in the google calendar to use it in external applications like the sunbird plugin for Mozilla's Thunderbird.

But the (german) help page does provide a general answer to address this problem:

Add a new Network → CalDav calendar and use this address:

https://www.google.com/calendar/dav/#contacts@group.v.calendar.google.com/events

Finally it should pop up a window where you provide your google account data (complete mail + password) and all the birthdays from your contacts should be shown in the calendar.