When you're training as an actor, you analyze the script and chart what motivates your character to say and do the things they do. "What's my motivation?" is the actor's cliché. The process always irked me because I think sometimes humans move in mysterious ways and we don't always know why we do things, no matter how much we analyze our motives. I liked to work with my actor's intuition rather than try to justify everything with the left side of the brain.
Why do birds sing? Well, most of it's about lurve and protecting and defining the home front. Male birds use their voices to tell mates they are lively and healthy and worthy suitors. They announce their presence to other males to keep them at a distance, defining territory with sound maps.
Bird calls are simpler vocalizations with many functions that ornithologists are still studying. One of the motivations behind calls is to keep in contact with mates, families, and flocks. Barry Kent MacKay in his book Bird Sounds: How and Why Birds Call, Chatter, and Screech, says that contact calls can help birds keep together in dark or foggy weather, keep the chicks close, or confuse predators. I am moved by his comment on contact calls: "Possibly for some species, it is a means of reassuring themselves that they are together and well, within a hostile world."
When you see a flock of birds, try to imagine what their calls are saying to each other. Why not observe humans and try to analyze our own contact calls. Perhaps twitter and texting are the new media for human contact calls. We enjoy the pleasure of checking in with one another, making sure we are nearby, safe, and not alone. It's the vocal version of comfort food. Humans crave contact on a continuum that depends on what kind of person you are (introvert/extrovert), what time of day it is (morning person?), and many other factors. We crave eye contact, physical contact, and vocal contact. One's psyche and spirit thrive on managing these kinds of interactions.