A user had this complaint:
"I'm moving Birthdays to a new calendar by dragging each event, one at a time, to the Birthday calendar. I've noticed that when I do so, the annual recurrence goes away. When I move the calendar item, I want the annual recurrence to go along with it. Is there an easy solution?"
Yes, moving recurring items in older versions of Outlook creates a single event and yes, there is an easy solution. Instead of moving the events one at a time from the day/week/month view, switch to a list view then drag the birthdays to the new calendar folder (or use the Move to folder command). Group by category and select all then move and you'll be finished in no time.
Outlook 2010 (and up) changes how recurring events are handled when moved. These newer versions will ask if you want to move the selected occurrences or the series. However, its still faster to move large numbers of events using a list view.