The language is just that. The phrases are exactly the same, whether three-day old chick or two-year old rooster. They run the gamut of sounds, like you've noticed, but they actually string together notes that have distinct meaning.
Most people know the "egg song" - rapid, excited clucking announcing The Product.
Close, but more frantic, is the "danger" call. It's similar to the egg song but louder, more frantic, and more rapid, degenerating into screeches. I've observed this when a bear or bobcat is attacking the pen. (Yeah, I'm screeching, too.)
The announcement "Here comes the human", is five notes, all the same, while the phrase announcing the cat or dog is the same five note phrase with the accent on the third note. When my days-old chick uttered the phrase for the first time, it blew me away since I immediately recognized it as the same as the adult version, only in a baby chick "voice".
Spend a lot of time listening to the chickens, you'll hear the phrases are distinctive. It's not just a lot of clucking and whistling. There's a barely audible trilling that they use with favorite companions that I'd love to discover the meaning of.
Some people report they once knew this chicken language as children, and that makes a lot of sense. Kids are sensitive to languages and meanings and pick it up easily. If your kids have spent a lot of time with your chickens, ask them what the noises mean. They know.
