iCal and Address book work well together. You can have iCal pull in all of the birthday information from your Address book, provided you have entered birthday info for some of your contacts. Be sure to add some 'Birthday' information into you contacts within Address Book.
Next, open iCal.
From the 'iCal menu', choose 'iCal->Preferences'.
Select 'Show Birthdays Calendar'.
Now, you will have a birthdays calendar in iCal. Whenever you update your address book and birthdays, the iCal Birthday calendar will be update too.