Google seems to be trying out something new. I was notified of my friend's birthday on the Google search page. I have this friend in one of my circle, so this seems to be because of Google+ rather than just search. Also i didn't get the notification anywhere else, but just on the Google search page. This means they are just testing it out first on the search page.
Clicking on the 'Say Happy Birthday' launches a Google+ post popup with just the friend as the recipient. So eventually we will be able to see our friend's birthdays, whoever might have shared it.