Dozens of Birthdays calendars suddenly appeared

Hi all,

Few days ago, multiple empty Birthdays calendars appeared on my iPhone 6s (iOS 10), and seemed to cause my Watch not to display calendars properly (I am guessing due to the high number of calendars, even if empty).

On top of that, more seem to appear every once in while, or at least turn back on.

I turned off all Contacts and Calendars sync (iCloud or gmail), but the ghost Birthdays still remain.

Also, these calendars do not show up on my other devices (iPad, Mac Book).

This iPhone was recently activated and set as new from an iPhone 5 backup which does not have these ghost Birthdays calendars.

Any ideas how to get rid of that?

Thanks