Yeah, I assumed most such tools do. Is that not the case?
(Amusingly, though, my own "how to keep track of birthdays" solution predates the addition of such fields to address books. When I learned your birthday, I would just put it in my (paper!) calendar, and transfer it annually.
Then when digital calendars happened, I just entered them there. To this day I have a calendar called "Annual Dates" that is comprised entirely of birthdays and other milestones worth keeping up with. Consequently and ironically, I don't use the in-address-book fields. Still.)