I have entered the birthday for several of my Contacts, and these birthdays show up in Calendar. I know that I can enable or disable all birthdays by selecting or deslecting "Birthdays" from Show Calendars in the top corner of the Calendar app. But I only want to see some of my contact's birthdays in the Calendar; for example, personal contacts but not ex-coworkers.
How can I prevent the display of certain birthdays?