In the past, to remove Google+ birthdays from your Calendar (and alerts on Google Now), you would have to remove all individual circles from the Your Circles group. However, the latest update to the Google Calendar app on Android added an option to toggle birthdays from appearing. It's as if Google realized that the "feature" was more of an inconvenience. Here's how to adjust the new setting:
Step 1: Make sure you are using the most recent version of Google Calendar on your Android device. This is denoted by 7-day week view, and version number 5.1. Unfortunately, Google has not updated the iOS version of Calendar yet, but it mentioned it would be coming soon.
Step 2: Open the Google Calendar app and head to Settings in the slide-out menu, then tap on Birthdays.
Step 3: Press the menu option for Google+ circles and contacts. In the pop-up menu, choose Contacts or None to remove the circle birthdays from your calendar.
What did you think of Google+ contacts showing up on your Calendar? Share your thoughts in the comments.