you could enter a birthday in Addr book at the bottom of the edit mode, under "Notes".
I find it better to keep track of events like birthdays and anniversaries in iCal, just enter them once, make it a yearly repeat event, plus you can set it to pop-up alarm or email you a reminder.
Sorry, too quick on the trigger.
I notice in the version of Addr book I have (4.xx) if I select from the top menu, "Card" I can then select to add a field from a list of many, one of which is "Birthdays".
To add 'Birthday' of an individual 'Address Book' entry:
01. Select 'Address Book's 'Card, Add Field, Birthday' sub menu item.
To add 'Birthday' for all 'Address Book' entries:
01. Select 'Address Book's 'Card, Add Field, Edit Template...' sub menu item.
The 'Address Book's 'Preferences' window will appear, with the 'Template' toolbar icon selected, and the window titled 'Template'.
02. From the 'Preferences' window's 'Add Field's popup menu, select 'Birthday'.