Why do some birds tweet and some birds screech?

Matt:

Why do some birds make song, where others hoot (owls), caw (crows), or coo (pigeons)? What is the explanation for such a range of sounds from similar species?

-- GC in LJ

What's the difference between a screech and a twitter? Maybe a few hundred thousand years. Birdologists consider the tweeters and warblers, the songbirds-- the birds we see around the lawn every day or hanging out in bushes or trees-- more highly evolved than the honkers and hooters. In addition to an evolution of size and color in birds, there was an evolution of habitat and, therefore, vocalizations. One of the main determiners of a bird's sound is where it spends most of its time. If you usually paddle around on a lake, you can see and be seen, so there's not much need to sing a complicated song to distinguish you from the other birds. A simple, loud quack! will probably do to attract a mate or defend your territory. If you're big and command the sky, ditto. A hawk doesn't need to sing an aria. But small birds lurk out of sight and are usually among other species of birds. To defend a territory or attract a mate, a small bird benefits from the ability to sing a more complicated song to advertise its species and itself as an individual. A bird's vocal machinery consists of a box-like syrinx low in its windpipe, controlled by pairs of muscles. The more pairs the bird has, the more versatile a singer it is. F'rinstance, vultures, storks, and ostriches have no working syringeal muscles, pigeons have one pair, songbirds generally have five to nine pairs.