How does Google know my birthday?

A few days ago, on my birthday, I noticed the usual Google logo had been replaced by one personally wishing me ‘Happy Birthday’. I thought this was pretty creepy because as far as I know I have never signed up for any Google extras such as GMail or Chrome, nor have I knowingly disclosed my birth date to Google. What is going on?
Hugh Searson, by email

You may be surprised by how much Google knows about you from what you tell it or reveal through your web searches, browsing habits, and the devices that you use to connect to the Internet. Information, stored as cookies by your browser are used to create a profile that can include your likely age, sex and lifestyle. You can see how much it knows, or thinks it knows, by going to Google Ad Preferences at: http://goo.gl/fuiu3. Specific information, such as your name and birthday, which is used to generate that personalised birthday ‘doodle’, would normally have been provided by you when you sign up to a Google service, so how it happened in your case is a bit of a mystery.

One possibility is that someone has created a Google Account in your name as a prank, to hide their own identity or for more nefarious purposes. It is not difficult to do, nor is your birthday a state secret and if you believe someone has set up a fake account you can report it at: http://goo.gl/MMP9k

Otherwise you may have signed up without realising or simply forgot all about it. This is also easily done. Google owns or controls dozens of products and services. In addition to GMail there is iGoogle, Buzz, AdSense, Google Drive, Picasa Web Folders, Google Voice and You Tube. You will also have set up an account if you have an Android smartphone or tablet. To find out if Google knows you go to the Accounts page (http://goo.gl/9Pbjq) enter your email address and any commonly used passwords. If it doesn’t sign you in follow through on the forgotten password or username links. If it transpires that you have a forgotten account you can delete it, change your settings and review the stored information through Google Takeout (http://goo.gl/2vLlr), which compiles a downloadable file detailing all of the services that have access to your data.