Hello @user82014 ,
Using a custom field won't make a difference. Only the default birthday field as built-in for the system will be able to trigger a birthday automation, and since there's only one per unique contact (email address), you won't be able to have multiple active on a single contact. In general, it's best to have only one person per email address, since unique email addresses are what determine unique contacts and associated insert-contact-detail functionalities for emails.
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