In addition to the calendars you create, you'll also see a special calendar called Birthdays. You can turn this on and off in the calendar view menu in the toolbar.
The Birthdays calendar is created automatically from birthdays listed in your contacts. It will show the birthdays of every person who has a day and a month of birth in their individual contact card.
For contacts with a specified year of birth, events will display the age they will be turning on that day.
Because this calendar is created based your contacts, it is not a calendar that is associated with CalDAV. If you're syncing your calendars via CalDAV, it is expected behavior for the Birthdays calendar not to sync.