-( This is chapter-specific and can only be found in: Chapter 4 )-
After receiving help from your Sentinel friend not once, but twice, to progress through this level, you'll be chasing after the fox once more. The second time the Sentinel helps you, it will crash through a wall to let you back in, and then the fox will be leading you up and around a fallen pillar. Near this pillar is a chipped part of the floor. If you navigate the camera around to look down beneath, you should see the remains of the bone bird from the second chapter strewn onto the stone.
For me, the trophy did not appear until I sacrificed the boy to jump towards the resting place. However, twice I have been made aware that simply moving the camera to keep it in your sights for an indeterminate amount of time will also unlock it for you. So, when you get to this point, move the camera to view it in different angles and it should unlock for you.