Grunt Birthday Party is a silver skull. It is the second skull on Crow's Bridge. After clearing the Barracks, you'll be sent out to meet the Arbiter. You'll drop down a short, narrow shaft at the end of a corridor and find yourself in a large tunnel with several pipes interconnecting from the left and right sides from which Drones will emerge-- they'll fly across from one pipe to the other in front of you.

The Black Eye skull is a gold skull. It is the first skull on the second level, Crow's Nest, and is in the same room you start in after the cinematic, high above your head atop a large pipe.

Getting to it is relatively easy. Walk to the end of the room, up a short staircase, and turn around and look for a green shelf. Jump up on that, up onto a girder, and from there up onto the large pipe. Walk back the length of the room to the end of the pipe nearest the video screens and claim this skull.