What is the meaning of the following quote? "Jack himself shrank at this cry with a hiss of indrawn breath, and for a minute became less a hunter than a furtive thing, ape-like among the tangle of trees."

At the beginning of chapter three we encounter Jack hunting in the forest. He has been unsuccessful in his endeavor thus far. His presence frightens a bird that flies off, uttering a "harsh cry" as it goes. The sound startles Jack and he pulls back. At this moment Jack reacts out of instinct rather than reason. The extract alludes to the idea that humans will respond to their deepest senses when threatened.
Jack is transformed into an animal. The hiss he utters is likened to the sound a snake makes when it is in danger. He becomes, for a brief moment, a "thing"—a creature intent on self preservation. He has momentarily lost the rational thinking skills a hunter would use when tracking prey. The reference to him being "ape-like" reinforces the image of an animal-like state. Jack has instinctively connected with his inner beast.
Golding alludes to Jack's savage nature throughout this section of the chapter. He describes him as being "bent double,"  "his nose only a few inches from the humid earth," "dog like," and "on all fours." Jack "crouched," "his bare back was a mass of freckles" and that his nostrils were "flared." All these descriptions can easily be used to refer to an animal and are used to accentuate Jack's inner barbarity. 
The use of such imagery foreshadows what is to happen later. Jack will indeed forgo reason and adopt a savage nature. He will paint his face and become a bloodthirsty hunter of not only pigs, but humans as well. He becomes ruthless and seems to have no conscience. He rejects the rules and creates his own tribe of savages. Also, when Piggy is killed, he expresses no sadness or remorse but sees Piggy's death as an advantage. With Piggy gone it will be much easier to hunt Ralph down. 
On a more universal level these images suggest man's own inner beast. Golding is demonstrating how our savage instincts can become a dominant force and rule our ability to reason. Such power can ultimately result in death and devastation. This possibility is aptly illustrated by the near-destruction of the island and the deaths of Simon and Piggy. 

At the beginning of Chapter 3, Jack demonstrates his tracking skills by crouching low to the earth and surveying the landscape for evidence that pigs passed through the brush. Jack continues to move through the forest as silently as possible so that he will not spook the pigs. Golding mentions that the forest was also silent to the point that not even the whine of insects could be heard. Suddenly, Jack accidentally startles a bird which cries out loudly through the forest. Golding describes Jack's reaction by writing,

"Jack himself shrank at this cry with a hiss of indrawn breath, and for a minute became less a hunter than a furtive thing, ape-like among the tangle of trees" (67).

Jack is also startled and upset that he spooked a bird which could possibly warn other animals that danger is nearby. According to the Merriam-Webster dictionary, furtively can be defined as something done in a quiet and secret way to avoid being noticed. Upon hearing the bird, Jack attempts to hide and become unnoticed, like an ape stealthily disappearing among tangled vines and trees. Essentially, Jack is trying to remain unnoticed because he is tracking pigs. After the bird cries out, he tries to blend in with the forest in order to remain hidden among the trees.