You can easily add birthdays to existing profiles.
Navigate to the People page
Use the search bar to find the person and click on their name
Scroll down on the left hand side of the profile and click 'Edit profile'
Scroll down on the right hand side and click on 'Birthday'
You can use the calendar date picker or type in the birth date.
*Use the year 1900 if you do not know the age of the person or the person has requested their birth year not be known.
Birthdays are built in Milestones. So any birthday you add into Notebird will show up in the milestones feed on the Activity page and within your Milestone Reports.
