How a bird gets the particular song that he sings differs among birds. In most groups, the sounds that birds make are believed to be inborn; birds like ducks, geese, loons, herons, and hawks, for example, have encoded genes that tell a young bird what kinds of sounds it is to make from the moment it hatches. Not so with the songbirds, such as larks, starlings, warblers, sparrows, finches, and many more, whose young birds must learn their songs from singing adults, much as we learn our speech from adult humans.
We often take this learning ability for granted, perhaps because we are so adept at learning our own speech, but the songbirds’ ability to imitate sounds is rare in the animal kingdom. This ability seems to be absent in our closest relatives, for example, the chimpanzees, gorillas, and orangutans. And it’s absent in the songbirds’ closest relatives, too, the flycatchers, including wood-pewees, phoebes, and kingbirds. Indeed, so rare is song learning among bird groups that it seems to have evolved independently only a few times, as in the ancestors of songbirds, parrots, and some hummingbirds.
Thanks to this song learning by songbirds, our listening pleasure is greatly enhanced. Although we can certainly enjoy listening to a non-songbird such as a Western Wood-Pewee, a flycatcher with inborn songs, he has a limited song repertoire consisting of only two simple variants of his song, and because the genes of these pewees are much the same everywhere, their songs are, too. What a contrast the pewee is to the mockingbird, a songbird that can sing one hundred to two hundred different songs. Because songbirds aren’t limited by how much can be encoded in their genes, some have developed impressively large song repertoires (though not all songbird species have taken advantage of that ability).
Song learning leads to all kinds of song variation that we can learn to appreciate and enjoy. Listen to different mockingbirds, for example, and you’ll hear how males differ in the songs they have chosen to mimic. Listen carefully to robins or individuals of almost any other songbird species as well, and you can hear how each bird sings with his own voice by varying his songs in either small or large ways from nearby birds of his own kind.
Each bird of a given species will be somewhat unique in the songs that he learns, but in most species he still conforms to the local dialect, just as we humans differ individually yet conform to our local speech dialect. (Native New Englanders, for example, often drop their r’s, as in “Go pa’k the ca’,” or they add r’s where not needed, as in “That’s a good idear.”) All of this versatility and variety among individuals and from place to place may make songbirds harder to identify but so much the easier to identify with.
A bird’s song depends not only on how he has developed it during his lifetime, but also on the genetic heritage passed down to him from his ancestors. Take thrushes, for example. Two species included in this book are the Hermit Thrush and Swainson’s Thrush; they sing very differently, but if we could follow the lineage of each species back through time, listening to how they’ve changed over a million years or so, we’d eventually come to an ancestral thrush, the single species that gave rise to our two modern ones. Since that ancestral thrush, why have the songs changed the way they did, so much so that we now have two strikingly different kinds of singers in the two species? Most likely, the two lineages somehow became geographically isolated, and females in these isolated lineages began to prefer slightly different singers. It’s likely that all the females were seeking essentially the same kind of information about the quality of their potential mates, but there are many forms that songs and singing behaviors can take to convey such information. Over time, the cumulative choices made by females in the two lineages were sufficient to produce two distinct species, each with its own sets of genes that dictate how to sing. As you get to know the birds in this book, pay attention to which species are closely related (especially those in the same genus), and you’ll start to appreciate the bigger picture of how or why each species has come to be as it is.
Young songbirds must learn their songs from singing adults, much as we learn our speech from adult humans.