Spring will be here soon. And with daffodils, crocuses and other signs of spring comes a burst of birdsong as males duke it out to get female attention. While the males trill loud songs, the females sit quietly, choosing who will be the lucky male.
Vocal male and quiet female songbirds are common in temperate zones, and have given rise to a common assumption. The best male songs get picked for reproduction, and this sexual selection results in complex song; females just listen and choose, so female song should be rare. After all, females don’t need to sing to attract mates.
But it turns out this commonly held assumption is not true. A new study shows that the majority of females of songbird species do sing, and it’s likely that the ancestor of modern songbirds was also a vocal diva. The results challenge the old wisdom about female songbirds, and suggest that when it comes to female song, it’s not all about sex.
Karan Odom, a behavioral ecologist at the University of Maryland, Baltimore County, has always been interested in birdsong. “As I began to study it in depth,” she says, “I realized there was a lot that’s unknown, and one area was the extent to which females were singing and the role that song plays in males and females.” Odom and her colleagues did a survey of 44 songbird families, going through bird handbooks and other sources to find records of whether males, females or both were singers. In results published March 4 in Nature Communications, they showed that female melodies are not rare at all. In fact, 71 percent of the species surveyed have singing ladies. So much for that quiet, retiring female bird.
The scientists then mapped the bird species on a phylogenetic tree, a family tree of sorts for a particular group of organisms. By putting species, or family members, in their correct places on the family tree, you can divine what their ancestors may have been like, even if you have never seen that ancestor, notes Mike Webster, an ornithologist at Cornell University in Ithaca, N.Y. “If you had a whole bunch of relatives,” he says, “some with blonde hair and some with brown hair, you map that on the family tree, and you can see that blonde hair originates with one particular person.” We may not know what great-uncle Moe looked like, but if all of his descendants had brown hair, there’s a high likelihood that he did, too.
Modern phylogenetic trees are often based on DNA from species that have been sequenced. Scientists can examine areas in the DNA where one letter might have been replaced with another. The similarities and differences between the letters can help determine how closely species are related.
The scientists took a phylogenetic tree of songbirdsand looked at each species, noting which had been observed to have female song and which hadn’t. By putting all of the song records on the tree to observe how closely related various species were, Odom and colleagues were able to show that the ancestor species of all songbirds probably had female singers. Kevin Omland, an evolutionary biologist at the University of Maryland and a coauthor on the paper, says that it is far more likely that a small and closely related songbird group lost female song than it is that 71 percent of all songbird species to gained female song through selection.
The scientists say that idea that female songbirds don’t sing probably arose not from the songbirds themselves, but from which species were studied and where. “When people first began studying birdsong,” Odom explains, “a lot of them were in temperate regions where not as many female birds sing.” This gave rise to the assumption that singing females were rare.
Marlene Zuk, a behavioral ecologist at the University of Minnesota, Twin Cities, is “always a fan of studies that examine commonly held assumptions.” She hopes the results of this paper will cause scientists to examine their ideas a little more. “Not everything is the same as in the temperate zone,” she notes. “There could be other implicit assumptions we are not aware of.”
Now that Odom knows more female songbirds sing, she wants to dig deeper to find out why. Females don’t have to sing to attract mates, but Odom hypothesizes that “maybe some other selection pressures are at play: to defend a territory or compete for resources.” With the new understanding of how widespread female song is, that’s a question that doesn’t have to be left for the birds.