It is probable that you have subscribed to Birthdays in iCal and that the birthday data is actually extracted from AddressBook data. The 'Birthdays' calendar is read only because it was not created in the iCal application.

If you need to delete or amend birthday data you should do it at the source in AddressBook. If you don't want any Birthday data showing in iCal then deselect the Birthdays Calendar or unsubscribe from it.