I use iCloud Calendar. Some time ago, I added my LinkedIn account to macOS Sierra. Then I noticed that Calendar showed not only the birthdays from my contacts (which I want to happen), but also all the birthdays from my 600+ LinkedIn connections. Sorry, LinkedIn pals, but I don’t want this.
If you find yourself in the same situation, there's a "Birthdays" calendar under the "Other" category in macOS Calendar. Uncheck or delete that one, as it’s one LinkedIn uses. You should also disable LinkedIn in your Contacts’ Preferences > Accounts category.