hi SWmmr Yes, now I come to think about it we probably do communicate verbally at greater length than any other animal. You ask do birds sing because thy don’t have predators. Well they do of course, and they can’t always fly away in time even from ground predators. Cats for instance, catch birds as often as mice.
reading your question has also given me another new thought. Birds are more ancient than mammals, evolving more or less, away from their reptile cousins, but reptiles do not make much noise. Indeed snakes don’t even hear high pitched sounds, only low reverberations. So did birds become birds because they chose to make noises?
In answer to your other question of whether birdsong is ‘for mating’, yes it is. The purpose of the song in song birds is to warn other birds to keep away from the territory of the singer, and his mate. In birds that do not share the bringing up of the family the calls are more specifically to attract a mate. The calls of animals have the same purpose, but since there are fewer animals where the sexes stay together to bring up the offspring their calls are fewer. Animals that share responsibility for offspring are much more vocal, and with a much more varied repertoire.

Probably less a "purpose" than function. Unless one considers birds were sitting around thinking a means to attract mates and terriotry and said – I think I’ll make some noise and see if that works."

Animal other than humans often communicate by means other than sound. Scents for example are often used more than sounds for communication even in animals that do use vocal signals as well: wild canines are a good example. In wolf packs, alpha females often can tell her pack members what to do by ear movements, teeth baring, even certain ways of moving the tail. This latter is believed to be communication by scent and sight! Sometimes predators have to be quiet so they don’t scare away their prey, but still those that work in packs have to be able to communicate to work together for food.
Some small lizards use color and a bulging throat to signify readiness to mate.
So coloration, dances, displays, motions, scents, and sounds we humans cannot hear are some of the communications going on in the animal world. If all these communications could somehow be converted to sounds, I would bet we would find a quiet forest noisier than a big city at rush hour!

Oh yeah, "less developed" is probably a misnomer. That would depend on what it is that one wants to develop. Are we really smarter in every way, we who have possibly used up and messed up too much of the world that is our only home and support? Gee, even a crow knows better than to soil it’s own nest…we see the earth as ours, but do we see it as our home, our food source, our water source, our nesting place? If so, we are not as intelligent as a crow, because we certainly soil our own nest, food, water…Maybe the animals are actually much much smarter in some ways than we proud, verbal, scientific, ruinous, humans can ever be?