Birds have amazing communication skills. You just have to listen to them singing to know that. But singing is only one of their ways of telling each other about the world.
Mother birds (or father birds, if they're one of the many species that allow the father to help raise the babies) can communicate danger to their chicks, by alarm chirping. This takes many forms, depending on the type of bird.
In song birds, it may be a harsh squawk, which brings other birds running (or flying, as the case may be) to help. Divebombing a hawk or other predator is their way of contributing to the war against predation of youngsters.
Sometimes, a mother bird will cluck or chirp continuously, as a beacon for chicks on the ground to follow, or as a comfort to nest bound chicks.
The chicks of peacocks, guinea fowl, pheasants, quail, grouse and turkeys will also do this, as a guide to the chicks location. They wander off, but Mom will find them.
Sadly, garden birds of all kinds will attack one of their own if it's hurt. This may be a defense mechanism to prevent a predator from being attracted to their area. Either they'll kill the wounded bird themselves, or chase it off.
Other ways that birds communicate is by fluffing up their feathers, and their posture.
Some of the most well known displays are by the prairie chicken, which has air sacs that inflate and deflate, making a booming sound. Only the male of this species does this, and it seems to be very attractive to the female.
Birds don't understand us when we speak. They do have good memories though, and watch us for body language that could be dangerous to them.
If you've ever been dive bombed by crows nesting in high trees on a city road, you may be aware of how alert they are and how quick to get on the attack.
Next time you are in the vicinity of birds, be aware of how they perceive you; as a threat, or a friend.