Where is this "other calendar" coming from on my iPhone

Solution 1:

The Birthday calendar is a “read only” calendar automatically created by iCal. You can change what will appear there by modifying your contacts’ birthday (in address book).

If you don’t like it, you can always disable it.

  • Open iCal
  • Go to Preferences:

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Solution 2:

I believe this comes from the address book. If you add a vcard with birthday information, it gets synced to iCal in this calendar. And then it might be getting synced to your phone.