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'.