Listen to the fluted chorus of a Wood Thrush, a beautiful song known to inspire artists and enliven eastern forests each summer. Now hear the gruff squawk of an American Crow.
Which is the songbird? If you said both, take a bow.
“Something can be a songbird and not be an impressive singer,” says Audubon field editor Kenn Kaufman. So, if singing ability doesn’t make a songbird a songbird, then what does? That question is actually a lot trickier to answer than it might seem.
The general public might throw the term around loosely, but for scientists, “songbird” has a more detailed meaning: It refers to a specific suborder of birds. All songbirds are perching birds, an order called passerines that share a distinct toe arrangement that helps them grasp branches. Passerines are separated into three suborders, the largest of which is Passeri. Birds in the Passeri suborder are called oscines, or songbirds. The suborder includes more than 4,000 species that range from the compact Golden-crowned Kinglet to the much larger Common Raven.
Despite their variety in size and musical talent, all songbirds do have something in common: precise control of a highly specialized vocal organ called a syrinx. Almost all birds use a syrinx to produce sound, but oscines have superior mastery of theirs. “The big difference is not the syrinx itself, but the muscles around it,” Kaufman says. “The oscines have a whole series of really complex muscles attached to the syrinx and it gives them much greater control.”
When a bird exhales, it can engage muscles inside the syrinx that control a series of membranes. As air flows over these membranes, they vibrate to create a specific tune. Because songbirds have the most control over their vocal organ, they can produce the most dramatic ballads.
Another trait that sets oscines apart is that, while other perching birds produce their songs from instinct alone, songbirds learn theirs. Research shows that songbirds hatch with a rough template of a song but need adult tutors to become expert vocalists. Young songbirds begin eavesdropping on their parents and other adults as nestlings. Some have a window of just a few months to learn their songs, while others, such as European Starlings, can add to their setlists through adulthood. “Songbirds have taken advantage of this ability to learn songs to produce very complex and large reportoires," says Donald Kroodsma, a retired ornithologist who studies birdsong. Other passerines only have a handful of tracks at their disposal.
That said, birds don’t always fit into tidy categories; Kroodsma's recent work shows that the Three-wattled Bellbird—a Central American passerine belonging to a different suborder—might also rely on learning to hone its tune, suggesting song learning exists outside of oscines.
While there is no authoritative definition of what constitutes a bird’s song versus its call or other noises, most experts agree on two key functions of birdsong: defending territory and wooing a mate. The better a songbird can show off vocally, the more likely it is to achieve its goal. In the Northern Hemisphere, male songbirds do most of the singing. Tropical species are more equally matched, as males and females often share duet performances.
Although most songbirds usually stick to the same basic template for a song, individuals occasionally throw in an additional note or a change in pitch. These variations might sound novel and exciting, and help draw more mates. “There’ll be an innovation, which is apparently quite attractive to females, and that innovation will then spread like wildfire through the whole population,” says Heather Williams, a biologist at Williams College. “Like anything trendy in human culture, they can shift things from generation to generation, or within generations for birds that keep on learning as adults.”
As a result, songbird melodies are always evolving. Most White-throated Sparrows in western North America used to sing Oh sweet Canada Canada Canada until a new tune took off over the past two decades. Now, they sing Oh sweet Cana Cana Cana.
On the flip side, some songbirds don’t have songs at all, says Kroodsma, who points to the example of a Common Raven. “It belongs to the songbird taxonomic group, but it really doesn’t have a song.” To make matters more confusing, there are non-songbirds with elaborate vocalizations. “I hear flycatchers doing a lot of things that sure sound like songs to me,” he says.
So, to recap, not all songbirds are talented vocalists or even sing, and not all birds that sing are songbirds. If you want to impress your friends, however, you can tell them that true songbirds are also called oscines, and they have exceptional control of a special organ called the syrinx. Or, instead, just relax, don't overthink it all, and enjoy nature's symphony.