Why would cats chatter when they spot birds? (Please read the details.)?

Obviously birds are a tasty treat that rate highly with our feline carnivores. However, how is it that cats are instinctively compelled to make a noise? Wouldn't that warn the birds that a cat is near? Or is it that cats only chatter when the birds are unaccessible and they wish to get closer? I am compelled to ask because our Tokinese has been eyeing some birds non-stop through the window for the past twenty minutes and I can see her whiskers twiddle as she chatters.