It is possible to see the birthday calendar by taking contact data when the account is Office 365, just as contacts are synchronized when being an exchange account
Normally your server will provide the birthday calendar.
But if they don’t, we do display birthdays from your contacts in the Agenda side-bar. Just click the settings cog next to the word Agenda, and make sure it is enabled.
Thanks Gary I hadn’t seen that the birthday calendar supplied by the provider appeared as a sub-calendar and I didn’t have it marked so I didn’t see it.