The Grunt Birthday Skull means that when you kill a Grunt will a headshot, you will see a spray of Confetti. It is found on the Maw Mission. When Cortana tells you to stop on a platform while you are escaping in the Warthog you will see Foehammer shot down. If you look over there you will see the Skull on railing to the right of where you are entering. You first need to use the Warthog to back up against the railing, then jump from the hog to the railing and grab the Skull.