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 options to both Enable and Disable Birthday Reminders.
After clicking one of these options, you'll see a dialog that just asks you to confirm whether or not you want to perform your selected action. 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.