You can create and send custom messages to your contacts on their birthdays using the automatic birthday e-mail sending feature. This is how it works:
1) Create your birthday message just like you would any other message in CAMPAIGNS > NEW MESSAGE
2) Select this birthday message as the one you would like to use in ACTIONS > BIRTHDAYS
3) For each contact that you would like to send a birthday greeting, set their birthday in their contact record. The birthday format is year, month, day. EG: 20041215.
Your birthday program will run every morning at 1am as soon as you select a birthday message. All of your contacts will be checked for birthdays on that day, and those that match will be sent your birthday message. You may use dynamic fields in the message as you would any other message.