Generally, cat chirping occurs when a cat is interested in or provoked by prey - a bird, a squirrel a rodent, and even a laser cat toy. Even wild cats can make this sound. You may notice your cat making a chirping or chattering noise while looking out the window at birds.
Next time you notice your cat chirping at the window, look outside. Are the cat's eyes glued to a bird, squirrel, or rabbit? Is her tail fluffed up and twitching from side to side? If so, your cat is anticipating a great catch! Cats are natural predators and even domesticated cats want to chase and catch the animals on the other side of the window.