The short answer is, they sing to live. But that doesn’t tell us much. It is not a testable hypothesis and is at too high a level of generalization to be helpful.
And before we go any further, we pause to note that we are talking about bird song today, not bird calls. Calls are the comparatively simpler sounds made by birds to stay in contact with one another, to call for mobbing behavior, or to sound alarms. (Alarm calls, by the way, tend to be high-pitched calls which make the source of the sound harder for a predator to find.) Today we are talking about the more complex, difficult, and – to our ears, anyway – melodious songs of the passerines and other birds that sing.
Birds produce song by forcing air through the syrinx, a bony structure at the bottom of the trachea. The syrinx resonates sound waves generated by the vibrating membranes of the syrinx. Changing the force of the air controls volume while pitch is controlled by both the force of the air and the muscular tension applied to the membranes. Some birds can even produce two separate notes at the same instant because they can control both sides of the syrinx independently.
The two leading bird-song hypotheses of our time are that male birds sing to attract mates and to establish and protect territories. Those hypotheses are at least testable, have been tested, and look to be correct – as far as they go.
But even they don’t get at the root of the question. Bird song evolved along with birds. As far as we know, the dinosaurs from which birds evolved didn’t sing. (Maybe that’s the reason they died out! Nothing to do with volcanoes in Asia or a meteor strike in the Yucatán: They couldn’t sing! Of course, neither can I, so I am not fond of my new hypothesis about dinosaur extinction. If true, it doesn’t bode well for my continued survival nor that of my children, who can’t sing either.) The first dinosaur-birds probably didn’t sing and certainly did not have the highly developed language of many current bird species.
But at some point we must assume the male birds that croaked out rudimentary songs had a sexual advantage over their competitors who hadn’t figured out anything more than avian karaoke. Otherwise, they wouldn’t be singing today
Said differently, why weren’t the ordinary, simpler, and easier-to-learn bird calls enough?
For that matter, why did the males of so many species develop their brilliant colors? That too takes more evolutionary effort than dull and drab. Moreover, brilliantly colored birds are easier for those predators which have color vision to spot. The same is true for song. A robin singing away in pre-dawn light is easier for a predator to find than if the robin was emitting only an occasional high-pitched call. Why does the robin take the risk? Natural selection, after all, destroys without fear or favor. Why is a noisy robin more likely to pass along his genes than one sitting quietly hidden two feet away?
The possibility exists that beauty plays a role. Maybe the bird that sings most beautifully is the one most likely to breed in spite of the danger? For those of you with a scientific/materialistic frame of mind, nothing excludes such a possibility: Beauty may be adaptive. How else to explain the Elegant Trogan? For those readers of a more spiritual/religious framework, why would beauty not be a survival requirement? How else to explain the Elegant Trogan?
But before we can go further we have to deal with “The Mockingbird Problem.” We’ll be back next time to discuss mockingbirds and their songs.
The photo of the fossil dromaeosaur at the American Museum of Natural History in New York was taken by Dinoguy2.