When you sync contacts, you are syncing the birthday info (if it is saved in the contact record). The birthday calendar in iCal is a special calendar that references the date information in the contact records and is nothing more than that.
For example, if you try and create an event in the birthday calendar, you get this warning:
You canxxx8217;t add or change events in the Birthdays calendar. Events in the Birthdays calendar come from your contacts in Address Book. To change them or add birthdays, use Address Book.