What are birds always squawking about?

Both wild and domestic, they crawl around, or perch somewhere, always making noise—sometimes at each other, other times alone as they eat seeds, other times perched high somewhere as if announcing something. Wherever they make their noise, birds singly or in small groups, are noisy.

What is it they are communicating? It seems somewhat mindless.

Commentary and humor welcome, but please stay on topic and maybe some serious answers or discussion will emerge as well.