I like the birthday feature, but I am not really happy about having to subscribe to two calendars in Lightning or any other client that can not generate its own birthday calendar.
Can the birthday events be integrated into the personal calendar? If not by some option in the GUI, it would be sufficient enough to use the “occ” command for this. E.g., if a command something like
occ synch birthday_calendar with personal_calendar for user X
be run once per day, that would be great.
That’s not possible yet, and maybe not ideal from a technical point of view. A separate calendar is easier to handle than modifying events in one big calendar table.
BTW, I don’t know how many devices you own, but if you have added the birthday calendar once on each device there is no more work to do on your site 
Well, it’s not just about me and my devices. In fact, I consider myself as an experienced user, and I know how to subscribe to calendars in NC, but still I would prefer not having to subscribe to more than one calendar. And think about this: you try to convince someone to use NC instead of Google or any other big cloud service, and then you got to explain this necessity for two separate calendars. That is something you don’t want to do every day.
Actually google has a calendar called Contacts that you need to subscribe to if you are using a third party app.
I prefer it that way as birthdays tend to clutter up an already busy schedule 