There are three separate approaches that restaurants take for providing specials to guess for their birthday parties.
First, there is the offer of free stuff for someone celebrating their birthday. They simply get their dessert for free or one item comped during a larger purchase. They simply have to show a drivers license proving their birthday. Sometimes, a restaurant will even take someone's word.
Second, being part of a larger group where the others are buying food as well, the restaurant will sometimes comp the birthday person.
Third, there is the offer to members of their loyalty program. Kind of like frequent flier programs, restaurant loyalty programs offer free items for their members and will often send special emails to their members reminding them of such deals. These don't always require that it be on your birthday, but instead can be just during the week or even during the month.
Rather than reinventing the wheel, there are plenty of places to get updated listings of free food offers. We decided to provide links to these websites that update their listings of free food. Here are just a few:
The Hustler Money Blog
Any of these websites, or many more like them, will provide you with a laundry list of options for restaurants that make offerings for birthday parties.
Our company, THEY improv, provides a variety of entertainment options that can be taken into these restaurants to take advantage of their free offers.