SELECT *
FROM  (
              SELECT Month(cust_dob) AS modified_date,
                     cust_id,
                     cust_fname,
                     cust_lname,
                     cust_dob
              FROM   birthdays)a;

OR

SELECT * FROM  (
              SELECT Month(cust_dob) AS modified_date,
                     cust_id,
                     cust_fname,
                     cust_lname,
                     cust_dob
              FROM   birthdays)a
WHERE  modified_date IN
       (
              SELECT Month(cust_dob) AS modified_date
              FROM   birthdays)  /* Use where conditions as per your requirement/*