Home & Garden Columns

Mockingbird Jazz: The Evolutionary Roots of Bird Song

By Joe Eaton, Special to the Planet
Tuesday July 25, 2006

I just finished a book called Why Men Won’t Ask for Directions, which despite the title is not another pop-psychology tract about gender differences. The author, Richard Francis, is an evolutionary neurobiologist, and the book is a rousing polemic against the sociobiologists and their intellectual heirs, the evolutionary psychologists: scientists who believe that just about every aspect of human behavior is an adaptation to something or other. 

Francis, on the other hand, is in the tradition of the late Stephen Jay Gould, who argued that some traits—physical and behavioral—are just byproducts of the evolutionary process, things that happened to be linked to other things that were targets of natural selection. Gould thought the bodies of all organisms were marked by “senseless signs of history”—like the “thumb” of the giant panda, jury-rigged from a wristbone. 

Francis does wind up with a discussion of alleged male-female differences in spatial orientation and related brain structure. But before he gets there, he introduces other creatures whose behavior or anatomy challenges the everything-is-adaptive model: sex-changing clownfish, parthenogenic whiptail lizards, Berkeley’s own spotted hyenas. 

(Yes, there’s a research colony of these very odd beasts in Strawberry Canyon. That’s a story in itself.) Along the way to humanity, he poses an interesting question: why does the mockingbird mock? 

On the face of it, the vocal performance of mockingbirds (there are several species, in the West Indies, South America, and the Galapagos Islands; ours is the northern mockingbird, Mimus polyglottus, the “many-tongued mimic”) looks like a straightforward case of Darwinian sexual selection. That’s the process in which the evolution of a trait, usually in male animals, is driven by some mixture of male competition and female choice. The classic example is the tail of the peacock: it’s not at all functional—it may, in fact, reduce its owner’s chances of evading predators—but the hens like it. 

Bird song has long been considered the audio equivalent of the peacock’s tail. Males sing to attract mates, and females somehow evaluate the quality of a prospective mate (good genes? low parasite loads?) by characteristics of the song. One characteristic supposedly selected by female choice is repertoire size. 

Was it Mae West who said: “I like a man with a big….vocabulary?” In birds like the marsh wren, there does seem to be a correlation between the number of song types in a male’s repertoire and his reproductive success. 

Marsh wrens are polygynous, though, like peacocks. Mockingbirds are monogamous. Why would a male mockingbird need his huge assortment of phrases, many borrowed from other birds, nonavian animals, and mechanical objects? (One tropical mockingbird is said to have learned the Brazilian national anthem.) 

Isn’t he a bit overdesigned? 

But, says Francis, what if the mockingbird’s repertoire is an evolutionary accident, one of Gould’s “senseless signs?” He explains that a typical songbird—a white-crowned sparrow, say—goes through three distinct phases in its song development. First, the bird produces a wide assortment of random sounds. Donald Kroodsma, a birdsong scholar, calls this “babbling”, analogous to what happens in human infants. Francis calls it the John Cage stage. 

Then comes “plastic song”: there’s some structure, but the song is still continuous and has an improvised quality. This is Francis’ Keith Jarrett stage. Finally, distinct songs crystallize out of the sonic mix: a single song type for the white-crowned sparrow, over 200 for the marsh wren. The bird sings that song, or songs—the “final song”—over and over for the rest of his life. In Francis’ typology, he has reached the Philip Glass stage. 

The songs of adult mockingbirds—and their near kin, catbirds and thrashers, and somewhat more distant relatives, starlings and mynahs—have all the hallmarks of plastic song. A few years back Rebecca Irwin, now at the University of Tennessee at Martin, studied bird song in terms of ontogeny (development) and phylogeny (evolutionary relationships). She suggested that the mockingbird’s song might not be an end product of sexual selection, but a quirk of the song-development process. If their ancestors were songbirds that went through all three stages, mockers may stop at Keith Jarrett.  

In support of that possibility, Irwin noted that other songbirds incorporate the notes of other species into their plastic songs. But these elements don’t survive into the final song. Mimicry may be part of the normal songbird learning process: although a certain amount is hardwired, birds need to be exposed to a model—a “song tutor,” either a father or a holder of neighboring territory—to get it exactly right. Mockingbirds, unlike white-crowned sparrows, are lifelong learners. Instead of settling on a final song they just keep noodling away, adding some new elements and dropping old ones. 

Those of you who were around in the ‘50s will no doubt remember Mad Magazine’s fascination with axolotls. In real life, the axolotl is a Mexican salamander that reaches sexual maturity while retaining its larval shape, including feathery red gills. Its relatives, though, grow up to be normal gill-less air-breathing salamanders. What happens to the axolotl is called paedomorphosis—and Irwin suggests that mockingbird song may be a paedomorphic behavior. 

Francis’s point in bringing up Irwin’s 1988 paper—which no one else seems to have followed up on—is that we can’t construct what Gould called evolutionary just-so stories for every trait. Evolution is about chance and necessity, and sometimes chance prevails. We are all, men and mockingbirds alike, the victims—and the beneficiaries—of a series of accidents.