So, it was about a group of about 8 walkers (6 true walkers and 2 whisperers, Lydia and one other guy). When Daryl hit the whisperer with an arrow in his leg, he dropped and yelled. Immediately, about 4 walkers turned walking forward and began to circle him to have lunch. Just from this scene alone, it seems the walkers' primary sense of hunting is still by sound? I wonder, if that whisperer dropped to the ground but help his pain without a noise, would the nearby walkers ignore him? BTW, is he a stupid whisperer or not? I mean, he was walking toward Daryl and he should have seen Daryl was shooting arrows. With the speed of the walkers, he would not have made it close to Daryl (regardless whether Daryl was shooting the head or other parts of the body). He and Lydia's leading a group of only 6 walkers approaching from a distance is a very stupid idea (unless they did not know Daryl has a long range weapon...but then, even without a long range weapon, just Michonne alone with her Katana can handle them easily at close range). A strange encounter there on the bridge.