My OSX calendar app is configured to show all the birthdays of my contacts (they are displayed as this special calendar with the parcel icon in OSX). It seems that iOS (6, on iPhone 5) does that too but doesn't recognize that this is the same calendar as the one in OSX.
As a result, when I sync all my OSX calendars with my iPhone, suddenly there are two birthday calendars, one seems to come from OSX and does not have the parcel icon. That calendar is called "Birthdays" when viewed in iOS. The other one seems to be the one intrinsic to iOS and just like in OSX has special parcel icon.
I would have assumed that the two apps of the two OSes should figure out that this is basically the same calendar. To get rid of the double events in my calendar view in iOS I can uncheck that "Birthdays" calendar in iOS so that the events of this calendar are not displayed. But that calendar still gets synched from OSX and I can't uncheck the "Birthdays" calendar in the OSX iTunes sync settings because there this calendar is not listed.
My question is now, how do I get rid of the redundant calendar on the iPhone?