You can add Contact Birthdays easily to your calendar using B2C from XDA Developers, a simple and free application that will scan you Contacts for existence of Birthday dates in their respective fields and if yes, adds the item to Calendar.

Usually when synchronizing with Outlook or Exchange, Contacts created on PC and having Birthday dates autmatically are added to your Calendar but that’s not the case with Windows Mobile. Download and install to Device, after which, you can run the app and select Add Birthdays; B2C will scan Contacts and add recurring event if Birthdays found.