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In a southern Indiana forest, Alexander Sharp craned his neck to monitor the Cerulean Warblers 50 feet above. He traced a male’s song to a nest at the edge of a gravel road, but soon realized it wasn’t the only sound he was hearing: An unusual call, similar to the Cerulean’s zeet but higher-pitched and repetitive, also came from high above.

It was mid-June, early in the breeding season, and yet the only explanation that Sharp—a former technician and current graduate student studying this species—could imagine was that a male nestling was attempting his first, unskilled song. The next day, Sharp returned with part of his research team from Ball State University, and they heard the same peculiar call. Then the bird peeked out of the nest, and they realized the sound’s source. It was a female, they were shocked to find.

Less than a week later, the researchers stumbled upon a second vocal female. The recent discovery, published last week in , is the first time researchers have recorded female Cerulean Warbler calls more complex than the zeet, which is common to both males and females. Bird songs are typically longer and more complex than calls—and the second female, they concluded, was truly singing.  

The Ball State University lab has been studying Cerulean Warblers for about 20 years, yet they had never observed females singing before. And while the team thinks that female song is rare in Ceruleans, overlooking other females’ vocals may be a common occurrence among bird researchers.

“Throughout the world there are a large number of species where females probably sing, but we just don't know yet,” says , an ornithologist who manages the . “For temperate regions especially, it may be due to a lack of people paying attention and watching for this behavior.”

In general, female songbirds are more likely to sing in the than in temperate regions like Europe and northern North America. Because of this trend, researchers in temperate regions may not be listening for female song or realize when it’s happening, Odom says.

The Ball State team credits their Cerulean Warbler finding to a mix of luck and close observation. “It makes you think there's probably a lot more discoveries to be made,” says Garrett MacDonald, the lead author of the new paper and a former member of the . “There's probably a whole lot more female birds in North America that sing that we don't know about yet.”

The two females’ vocals () were different from each other, from the males’ song, and from every other species’ the team could think of. The first female chirped a song-like variation of a zeet call, mostly as an encore to her mate’s song, according to the paper. The second female performed a truer song: long, rhythmic, and complex with frequencies and harmonics. She only sang while foraging near her mate. 

Because of the mates’ presence during both song-like calls, , a biology professor at Ball State and head of the research team, thinks the females used the calls to strengthen their relationships with their partners. On the other hand, “it could very well be anomalous,” Islam says. “The calls may not necessarily have a function.”

To produce their vocalizations, the females may have tapped into an ancient blueprint for song, passed down from a female ancestor that sang more often, Islam says. That would contradict showing that the ancestor of New World warblers (including Ceruleans) probably didn’t sing. But this conclusion could be overturned if researchers find new evidence of song in more female warblers, says , an ecologist at the University of Northern Colorado and an author of that paper.

While still unusual, Cerulean Warblers aren’t the only warblers with a few females in the choir. In , a female Hooded Warbler performed a unique song for over a month, more frequently than even most males in her population, according to a 1994 thesis paper by then ecology graduate student Lesley Evans Ogden. And at least one (and possibly three) female Prothonotary Warblers have been caught singing that differ from the male song.

“It’s important to put these findings out there because you might encourage others to look for female birdsong,” Benedict says. “Once you start looking for something, you might realize it's not as rare as you think.”