You can learn more about tracking contact Birthdays in Redtail and Birthday Reminders here. In this post I just want to cover how you would go about enabling or disabling Birthday Reminders for multiple contacts in bulk.
From your Advanced Search page, after running a search that will include your desired contacts, place checks to the left of the names of those contacts for whom you want to adjust their Birthday Reminder settings:
Then, click the Contact Options menu to the right of your search results and select Bulk Actions:
In the resulting modal, you'll see the option to Change Birthday Reminders.
After clicking, you'll see a dialog with an option to Enable or Disable Birthday Reminders. After confirming, this change will be reflected within your database.
As you can probably see, it's a quick and easy method to change the Birthday Reminder settings for multiple clients at one time.