A sparrow, a cardinal and a tufted titmouse wait for their turn on a bird feeder in Maryland. (Linda Davidson/The Washington Post)

Only male birds sing. For years that was the assumption among amateur birdwatchers and ornithologists alike. After all, male birds are “the obvious ones,” says Lauryn Benedict, a biologist at the University of Northern Colorado. “They're out there showing off, strutting their stuff.”

But Benedict and fellow birdsong expert Karan Odom, a biologist at Cornell University, want you to look closer if you hear a chirp or warble. Female birds are not, on the whole, silent. In a call-to-ears published Wednesday in the journal the Auk, the two scientists say that “birders and researchers need to be aware that female birds regularly sing, and they need to take the time to evaluate the sex of singing birds.”

The tipping point for Odom came in 2014, when she concluded that birdsong is an ancestral trait shared by both sexes. Female birds sang in 71 percent of 323 species surveyed, she and her colleagues reported then in a Nature Communications paper. They traced this behavior through the bird family tree, winding back the generations to a common singing ancestor. At that point in history, they wrote, both male and female birds sang.

Benedict, who was not involved with that work, described it like this: Instead of males evolving to be loud, “females have evolved to be quiet.” 

To be fair, there is a reason singing birds were assumed to be male. A geographic bias exists, Odom said. In temperate regions like North America and Europe, where the bulk of western ornithology takes place, many female birds do not sing. Instead, it is the migrating male birds who show up and strike up a tune to stake out a spring and summer territory. And, of course, they serenade females with the goal of mating.

But in the tropics, where there is a rich diversity of bird species, both male and female birds belt it out. (A few hypotheses suggest why female tropical birds are singers. For instance, tropical birds often live in the same area year-round rather than migrate north. If so, the female birds must sing to claim territory as their own, just as the male birds do.)

Odom and Benedict said they encourage birders to listen, then look. In some species, it is easy to distinguish male singers from female ones. Consider the cardinal: Males are bright all over, as if dunked in red paint; females are tan to brown with only flashes of red on their wingtips. And, yes, female cardinals sing. Other bird species do not offer an easy visual identifier, however. So with those around your house, watch if a singer performs “quintessentially female” behaviors for that species, Odom said, like building a nest or incubating eggs.

Identifying female birdsong can help researchers in several ways, Benedict said. In the lab, birdsong plays an important role as a biological model of communication — birdsong is learned, and it has syntax and different ways of delivery. Neurobiologists might be missing a trick if they only use male songbirds.

In the field, listening to birds is a noninvasive way to track population health. Citizen scientists and birdwatchers play key roles by recording observations at databases like eBird and iNaturalist or uploading birdsong audio clips to a website created by the study authors or the Macaulay Library, the two scientists said. They recommend observers make a note of the bird's sex or clarify when its sex is unknown.

“If you see a singing bird, just don't start by assuming it's a male,” Benedict said.

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