Google Contacts' birthdays not showing up in Apple Contacts or Calendars

As a primarily Gmail-using guy, all of my calendars and contact information is stored with Google. All my Google Contacts are exactly the way I like them, with various information such as nicknames, birthdays, important dates (my anniversary), etc. I just bought an iPad Air 2 and proceeded to sync my Google information as described here: https://support.google.com/mail/answer/2753077?hl=en

All my contacts were successfully added. However, for all of my contacts that did not have a Google+ account, their birthdays, which I entered manually into my Google Contacts, were not synced. None of my contacts on my iPad have their birthdays, either in their contact card or the calendar, even though I explicitly have their birthdays in my Google Contacts on Gmail.

Is this a known issue? Is there a step I'm missing, and will I be forced to manually enter all these birthdays?