When you hear a bird warbling, you probably think the crooner is male. And chances are if you’re in the Northern Hemisphere, you would be right. But females also evolved to sing, and many still do—although generally less than the males. One reason may be that it’s more dangerous for them to sing, especially when nesting, scientists report today. At least, that’s the case for female fairywrens, the most vocal of which are the most likely to have their eggs and chicks eaten.
The study “provides some of the first field evidence indicating why females of so many songbird species might have lost song,” says Karan Odom, a Ph.D. candidate at the University of Maryland, Baltimore, and the lead author of a 2014 study on the evolution of birdsong.
Female superb fairywrens (Malarus cyaneus)—a small Australian species—aren’t the only female songbirds that sing. In fact, females sing in 71% of songbird species, often for territorial defense. In species like the superb fairywren, some females even sing when they’re in their nests, a place where, at least theoretically, they should pipe down so as not to attract predators. Rodents, birds, cats, and foxes have all been seen preying on the fairywrens’ nests. “People had observed [this singing in the nest behavior], but they hadn’t investigated it,” says Sonia Kleindorfer, a behavioral ecologist at Flinders University in Adelaide, Australia. “It struck me as odd, and very risky.”
But males also sing near their nests. To find out whether female singing poses a greater risk to their offspring than male singing, Kleindorfer and her colleagues monitored the eggs and chicks at 72 wild superb fairywren nests, along with female and male singing, for two nesting seasons in 2013 and 2014. The scientists marked a nest, a domed structure of woven grasses close to the ground, as “attacked” if the eggs or chicks vanished before they were capable of fledging, a developmental period requiring at least 25 days.
Adult superb fairywrens are socially monogamous, and both males and females produce what scientists call a “chatter song” to defend their territory. They learn the song—which they sing solo and usually away from the nest—as fledglings. The song consists of roughly eight vocal elements repeated about 50 times for 3 seconds. The female’s chatter song is shorter and a bit less complex than the male’s, but otherwise they are identical. Both sexes also produce other songs and calls, but when the female is inside or near the nest, she twitters only the chatter song. And she does so in response to her mate, who sings this same tune after returning to their territory from a foraging or other trip.
“Songs can have multiple functions,” Kleindorfer explains, “and in this case, it’s likely about strengthening the bond” between the pair. “He’s letting her know, ‘Hey, I’m here.’” He may then fly away again, or stay to guard the nest from a nearby bush while she forages. Males do not incubate the eggs, but they do feed the chicks.
Males and females sang the chatter song more often when they were just beginning to nest, the team found. Both sexes continued to sing this tune when they had eggs and chicks in the nest, but at a lower rate. There was a key difference between genders: The males sang away from the nest, whereas females sang much closer to or inside it. Such singing carried a big cost: It attracted predators, the researchers report online today in Biology Letters.
The scientists backed up this finding with an experimental test. They baited artificial nests with quail eggs and broadcast female chatter songs at low (six calls per hour) and high (20 calls per hour) rates. Predators ate the eggs at 40% of the high call rate nests, but at only 20% of the low call rate nests.
Because some female superb fairywrens sing when they are either inside the nest or close to it—instead of farther away as do the males—they inadvertently reveal its location to predators, the scientists say. “That’s a high cost for singing,” says Kleindorfer, who adds it is “clearly” creating evolutionary pressures on the females. Indeed, females never initiated the chatter song while inside their nests, instead only singing in response to their mate. And half of the females studied (six out of 12) never uttered one twitter while inside or near their nests—a difference that may be due to learning or some other factor, because singing is a variable behavior, not a fixed trait. That means that the female fairywrens (and songbirds in general) are not programmed to sing, but can change when, where, and how often they croon in response to what’s happening around them.
The study “is important because it identifies a specific potential cost of singing for females: attracting predators to the nest,” says Jordan Price, a behavioral ecologist at St. Mary’s College of Maryland. It may be, he adds, that this is why the females sing far less than their partners or not at all in other songbird species in which females provide more parental care.
The study also helps overturn an old idea about why male birds sing whereas many females (primarily species in northern temperate environments) don’t, Price says. “Male song is often cited as a classic example of a sexually selected ornament,” he explains, meaning that until recently scientists considered the male birds’ tunes to be much like their flashy feathers: a means of attracting mates. Supposedly, males sang while females only listened. But this study shows “that sexual differences in [male and female] song production are due to natural selection against singing in females,” instead of being the result of “sexual selection for singing in males.”
“It’s only one study, and doesn’t necessarily explain every species where females no longer sing,” Kleindorfer adds, noting that other behaviors or life history differences might also lead female birds to sing less or not at all. But based on her findings, it seems likely that in some species, female birds gave up their songs because it was risky and attracted predators. “They don’t sing not because they can’t or don’t want to, but because they shouldn’t,” Kleindorfer says. “It’s too dangerous.”