Hide birthdays in BB Calendar app?

My BB (Android) calendar is cluttered with birthdays. I don't know where they're coming from, also because the event colour (green) does not match with any of my synced calendars. I suspect that they are automatically taken from the Contacts app. How can hide these birthdays?

Could they be coming from Facebook? I have that all the time, where Facebook friends' birthdays populate my calendar.

I don't think so... I actually have the app "Facebook Calendar Sync" installed, but I hid the birthdays in the calendar app and when I make it visible again I get the birthdays twice. Also, it has a different colour (blue instead of green). Furthermore, I'm getting some birthdays in my calendar of people I don't have on Facebook.


Edit: I think the birthdays are coming from my Google account, because they're gone when I disable it. However, there is no birthday "sub-calendar" in the Google account (only "events" and "week numbers"), and the web version of Google calendar doesn't show any birthdays. Also, the birthdays in the BB calendar are marked green, while the Google events I added myself are red. Weird.

Edit 2: I am now very sure what's the reason. There is this un-deletable "birthday" calendar on Google which is created from my contacts (also on Google). On the web version I can make the birthday calendar invisible, but I suppose that BB Calendar didn't get the memo and still shows it to me. No solution yet.

After I made the birthday calendar visible on the web-calendar again, it appeared on my BB calendar settings with a green colour as "Contacts". However, the font is grey and I can't delete/hide it. I can now set it as invisible in the main menu (pressing the three lines on the top left), but the birthdays don't actually disappear

Edit 3: that's still not it. when I unhide the Google birthday sub-calendar, it just appears a second time. I think there is some kind of sync with the contact...

I think I have the same problem.
Attachment 424642
I have a seperate calendar within the google calender which is called "Birthdays" (in german: Geburtstage). That calendar has the color pink. But on my DTEK50 I have an additional event for every birthday (-> green square), which I cannot delete. These events are not shown in the browser or anywhere else. I do not have facebook installed and I deactivated google+ completely. Is there any possibility to get rid of these events?