Social networking sites are probably the first resource that you can check for a birthday. In fact, with just an email address, you can find someone on Facebook and access what they've posted publicly online.
Search for Someone with Google: A simple Google search can bring back a surprising amount of information, including names, addresses, phone numbers, and birthdays.
Fifteen Specific People Search Engines: Search engines are great, but there are specialized search engines you can use to track someone down. A Step by Step Guide to Finding Someone Online: If you're not successful tracking down a birthday, this step by step guide can help you walk through finding little pieces of information all over the Web.