Clucking Chicken

Sound is caused by the vibration of the particles in materials.  When objects collide or move through the air, they cause the molecules around them to vibrate.  In many cases, the vibrations are transmitted to air molecules, which then vibrate causing a wave of sound to move across an area.  Eventually, the sound wave may reach an ear where it causes the parts of the ear to vibrate and the person to sense an audible sound. 

Like people, chickens use sound to communicate.  For example, roosters will crow to mark their territory, warn their flock about predators, and in anticipation of the sunrise.