In case you haven’t noticed yet:
In case you use a Browser which “is registered with Google” (cookie, Chrome sync, Hangouts plugin …) in a way that Google knows who you are when you enter the search page, and you have your birthday entered in this account, then Google presents you a “Happy Birthday” Google-Doodle on that day. If you are not on Google+ and you klick on the picture, you get a page which offers you to upgrade to Goolge+.
Funny/scary? Up to you. For me it just visualises what I knew already (you can do a lot with personal info, and Google get’s a lot of this) and I say “Thank you Google” (but I don’t upgrade to Google+).