When you hear birds singing...what are they usually singing about or for and why?

They have alarm calls (sometimes several types, one for a hawk and one for a ground based predator such as a fox), territorial/mating calls (this is my turf, males stay out but females are welcome), and other types of sounds that they make. For example, a female may respond to the male's mating call with a call of her own to acknowledge she is receptive to his courtship. A group of migrating geese may call to keep in touch with one another. There also appear to be calls of ecstasy that birds may make moments after mating. A nestling vocalizes to the parents to be fed. Then there is a phenomenon called dawn chorusing, during which a large number of birds may vocalize, as though they were saying, "rejoice, it is another day, and it is good to be alive." And then some jays may imitate the call of a red-tailed hawk to scare other birds away from their food. Other birds may fake alarm calls to achieve the same goal. Other birds may mimic other sounds around them, including those coming from lawn mowers. Only birds will fully understand all of the reasons they make sounds.