do you have all of their birthdays memorized

I surely do. It's not a problem to memorize a few numbers related to the people significant to me enough to send them a birthday message.

The Dunbar numbers go around 3-5, 9-15, 30-45 [0], so even "the 2nd tier" relationships' birthdays are not really that hard to remember

[0] https://royalsocietypublishing.org/doi/abs/10.1098/rspb.2004...