Starting in January 2026, you can now use Chimpegration to take the birthday stored within a Raiser's Edge record and place it within Mailchimp's Birthdate Merge field.
The Birthdate Merge Field in Mailchimp can be formatted to accept birthdays in the form of day/month or month/day. If the incoming value from Raise's Edge includes a year as well, Chimpegration will know to strip out the year.
To apply the birthdate to Mailchimp from Raiser's Edge, run a Mailchimp Data Update process and when you get the Field Mapping step, be sure to map the Birthdate field from Mailchimp to the appropriate Birthdate field from Raiser's Edge.
Birthdate is included as part of standard fields included in Constituent Lists. If you are using a Query with the Mailchimp Data Update process, you will need to have included 'Birthdate' as an output fields on the Query you are using.