Which muscle is responsible for moving the chicken's head left or right and moves the scapula?

The trapezius muscle plays a crucial role in both moving the chicken's head from side to side and in the motion of the scapula. This is attributed to its anatomical location and function; the trapezius extends along the back and the sides of the neck and upper back, which provides the necessary leverage for both head movement and scapular stabilization.

Its fibers are oriented in such a way that they can contract to facilitate lateral movements of the head, allowing the bird to look around effectively. Additionally, since the scapula is closely associated with the movement mechanics of the wings and upper body, the trapezius assists in anchoring and maneuvering this area during flight and other movements.

In contrast, the other options do not directly correlate with the actions specified. For example, the flexor perforans is primarily involved with forelimb movement rather than the head or scapula. The obliquus muscles are more related to the abdominal area and are not responsible for the head's lateral movements. The savorius muscle, being less well-known in the context of poultry anatomy, is not specifically recognized for any relevant function regarding head or scapula movement. Hence, the trapezius is indeed the most appropriate choice for this question.