This is Charlie and like a lot of cats he makes a very peculiar chattering noise when he sees a bird. Some experts believe that these special noises are due to the frustration of not being able to get to the prey. Others believe that the noises are strictly due to excitement.
I like to think that Charlie is experiencing delight and eagerness at seeing a feathered friend through the window and even begins to pace around. Maybe he is venting his extreme irritation with the fact that he is unable to perform as a hunter when the prey is so very close and not mumbling to himself like his human thought!