A Birthdays Only Calendar 
To keep your calendars from becoming cluttered with events and birthdays, you may wish to setup one calendar for just displaying birthdays.
1Click Calendars & Moderators->Calendar Manager.
2Click [Edit] next to the calendar you wish to setup this way.
3Click <Yes> next to Show Birthdays on this Calendar.
4Click [Save]
5Click Calendars & Moderators->Calendar Permissions.
6Find the calendar you with to modify and choose the usergroup you wish to deny posting events for (probably the Registered Users group) by clicking [Edit]
7Make sure Can Post Events is set to <No>.
8If it is not, select Use Custom Permissions, then select <No> next to Can Post Events.
9Click [Save]
Note:
If you have any events on this calendar you will need to move them to another calendar or they will still appear.
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