the birthday on the calendar gets synced from your contacts. I suggest you check your contacts to see if the birthdays are recorded correctly in the appropriate contacts
Sychronizing my iPod Touch (SW-Version 3.1.3) with iCal (V. 5.0.2) (iMac, OS Lion 10.7.3) via iTunes following happened:
All calendars were synchronized, but the Birthday-Calendar was not.
I reinstalled the Software of my iPod Touch and synchronized again, but the Birthday-Calendar kept away.
Anyone out there having an idea, what can be done?