Found the following article online, use it to answer the question: What animals' flocks make a quiet chattering sound while at rest?  The territorial call of the Australian raven is a slow, high ah-ah-aaaah with the last note drawn out. It uses this call to communicate with other Australian ravens in the area. When giving this call, the species has a horizontal posture, holding its head forward and body parallel to the ground, while perched on a prominent position. It ruffles its hackles and lowers its tail, and sometimes holds its beak open between calls. In contrast, the little raven and forest raven hold their bodies in an upright posture. This call becomes louder if trespassers encroach upon the Australian raven's territory. The five Australian species are very difficult to tell apart, with the call being the easiest way to do so, although the drawing-out of the final note—long held to be solely recorded for the Australian raven—has been recorded for the other species and is hence not diagnostic.The volume, pitch, tempo and order of notes can be changed depending on the message the Australian raven intends to convey. There is a variety of contact calls: a pair often makes a low murmuring sound when preening each other while roosting, and members of a flock carry on with a quiet chattering while at rest. Birds make a call and answer sequence if temporarily out of sight of one another while foraging. Birds in flocks make a single high-pitched caa while flying over another territory as a transit call to signify they are just passing through. An Australian raven will give a longer caa with a downward inflection to signify its return to the nest to its mate.
Answer:
Australian raven