In this scene, Brian begins to think back to the plane crash, he remembers the pilot passing out and kicking the plane's rudder pedal. He knows that now that the plane has gone off course, the people looking for Brian will not be able to track the plane's path. This shows that Brian may be stuck in the woods forever and he starts thinking long-term survival.
Brian, desperate for shelter, begins to look around and finds an opening in a cave near a waterfall. Brian claims this as his new home because he can move another rock in front of the cave door so he can be safe. This is remarkable because Brian finds a place to sleep, eliminating the possibility of being killed by a bear, which is one of Brian's biggest concerns.
Brian, growing hungry, notices a cluster of brush with birds flying around it. He goes to check what they are doing and finds them eating berries. His hungry takes over and he eats the berries, causing Brian to get sick and throw up again. This shows that Brian needs to think more about his decisions and think with his mind over his stomach.