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.
Using iCal 5.0.3 in Lion. Maybe a week or so ago I started getting this error when trying to delete birthday entries:
Only the calendar administrator and any delegates with permission can make changes to read-only calendars. To make changes to the calendar, contact the calendar administrator.
This is a home computer and I'm logged in with an Admin account -- the only account we ever log in with. I'm not aware of any software changes unless something happened with an Apple update. Any ideas?