Biologists at UMBC were on the hunt for female songbirds. The world sat up and took notice when they found them.
By Joel N. Shurkin
Illustrations by David Plunkert
Once upon a time – 60 million years ago or so in Australia – a small, perhaps brightly feathered descendant of the recently extinct dinosaurs opened its beak and produced a melodic set of notes. And while it is impossible to know if the creature was startled by the sound it produced (more likely the change was subtle), that moment was the prelude to all bird song.
Kevin E. Omland, professor of biological science at UMBC, and Karan Odom, a doctoral student in Omland’s lab, are among the scientists taking a closer look at bird song – and reshaping the contours of received knowledge about it that dates back to Charles Darwin’s observations and beyond.
It’s a deliciously complicated problem. While scientists have known for a while that songbirds first evolved in Australia, Omland and Odom have used that fact to disprove a theory of Darwin’s and a common myth about birds: Unlike what most people think, female songbirds sing just as much as male birds do, and may also be just as colorful.
A recent paper with Odom as lead author and Omland as one of the co-authors, detailed the wide extent of female bird song, and garnered media coverage around the world, including the BBC World Service and Australian radio.
“This is not a well-studied area. They are looking at a question that has not been looked at a lot,” explains Michael Webster, a researcher at Cornell’s prestigious ornithology department.
Bird song emerged from descendants of recently-extinct dinosaurs.
Humans have been fascinated by bird song for as long as they have had ears to take it in. And for almost 200 years, theories about bird song would suggest that the proto-songbird who made that first distinctive chirp was male.
The notion of mute, dull females and gloriously colored males in part goes back to Darwin’s observation of Galapagos finches during his second voyage on the HMS Beagle. In The Origin of Species, he later wrote: “Female birds, by selecting, during thousands of generations the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect.”
It was an easy assumption for Darwin to make: songbirds in his native England generally behaved that way as well. In other words, the males competed for females by trying to be the most beautiful and songful, and the females selected on that basis – sexual selection. Think of strutting male peacocks or birds of paradise doing hilariously elaborate dances, for example.
Omland, Odom and their colleagues and students say that this is an incomplete explanation. Darwin and everyone else had it wrong, and why they did so is a matter of geography, not biology.
“A lot of the research on bird songs has been done in North America and Europe,” Odom says “where you see brightly colored males and they are singing and they are singing for a short period of time because they have to breed quickly and migrate.”
In the tropics, it turns out, that isn’t true, she says. What most of temperate zone ornithology was studying was not typical of the world’s birds, particularly songbirds.
In most parts of the world, like the Amazon, the Congo, the rain forests of New Guinea, the observer cannot tell the difference in coloration between the males and females. Female birds are just as likely to sing as male birds and can defend territory just as male birds of some species do.
Omland’s research shows that bird song in fact depends on where the birds were and the kind of life the birds lead. Even birds like the orioles – with whom we live with happily in the Maryland area – have species that vary in behavior, depending upon where they are and their life style.
Odom, a Florida native, studies a species of fancy orioles called Troupials in Puerto Rico for her dissertation. Undergraduates from UMBC and the University of Puerto Rico work on another species, the Puerto Rican Oriole. Back in Maryland, Omland specializes in the Baltimore and Orchard variety of oriole.
Orioles who live in Maryland in the summer follow Darwin’s pattern. The gender rules are quite distinct. They live here for only a few months and then migrate south to Latin America before the snows come. It is easy to spot a female: She is yellow-brown and she does not sing a complex song. The males are the typically black and orange. But orioles dwelling in Puerto Rico – same genus, Icterus – have a different life style.
Ornithologists think singing is a way of declaring presence and defending territory. When you are a migratory bird, time is of the essence and the singing helps with defense.
The Maryland orioles have a time problem: They have to attract mates, reproduce and then leave before winter comes, Omland says. The males arrive first. In songbirds, it is the females that build the nests. So the males need to grab the attention of the females as quickly as possible. Their clock is ticking, so the males tend to use coloration and song to attract mates. That is the sexual selection that Darwin found in his research.
If you live a more sedentary life in the tropics, it is less crucial, or at least it plays out in a different way. Female birds in the tropics play a larger role in the competition for resources. They may even be defending mates. There also is more competition among females in the tropics.
In Troupials that Odom studies, males and females look the same, behave the same way, and males and females sound the same. Usually it is the males who do the “dawn chorus,” and the females who do most of the daytime singing.
Orioles who live in Maryland in summer follow Darwin’s pattern.
To do her research on Troupials, Odom sets up camp in a dry forest, mostly distinguished by tall grasses and cacti and sharp-leaved plants worth avoiding.
On a typical research day, Odom and her team get up at 4 a.m. and go out in the field by 5 a.m. to be ready for six hours of observation. By late morning, it’s too hot to work so the researchers go inside, rest and have lunch.
In the afternoon, they follow the birds to wherever they “want to take us” and catch some with traps. Oranges are the main lure. Orioles love oranges. The birds are sexed and then banded by gender, and then let loose unharmed. Females get pink bands, plus an aluminum one for identification.
Odom is studying how these Troupial orioles use their songs. Why are they singing? Do they have different songs? They can sing in chorus or have a “sing out,” with one bird singing, followed by another, as if they are planting stakes in the ground to delineate territory, or simply making an existential declaration, “I sing therefore I am.”
The orioles she studied have such stability and routine that she identifies them by where their nests are located. There is a “greenhouse pair,” and a “visitor center pair.” They can all be found in a 100-to-200 meter square area.
Once Omland came to visit and Odom’s camp and asked to see a particular family and she told him to stand by a certain tree and they would show up. They did.
The birds don’t migrate, and they live together for years. In those species, both males and females are colorful, and both sing. The only way scientists can tell the difference in gender is through DNA analysis. How the birds tell the difference is unknown, Omland says. Other species of orioles in the tropics are also like that.
“Both sexes in the tropics are very progressive,” Omland jokes. “Both sexes wash the dishes. Both sexes defend the territory.”
In 2009, Omland and an international group of ornithologists published a paper suggesting that the changes over time had occurred in females, at least in orioles. The difference in the genders happened because the females evolved through time. The males were always brightly colored and always sang. A remaining question for scientists is why the females changed.
Darwin’s observation on bird song drew an incomplete picture.
The question of how many species of birds have females who sing was particularly interesting. Is what UMBC researchers and other ornithologists found in Puerto Rico typical in the rest of the tropical world?
Odom and her colleagues did a massive study to find out. The results, published in Nature Communication last summer, made headlines in newspapers and magazine worldwide, including Germany, Holland, China, South Africa and Great Britain.
“Some of the reasoning for our study is that people have known for a while that songbirds came from Australasia and that in that area and just in the tropics in general, a lot of females sing,” Odom says. Birds like Australian magpies and butcherbirds sing, sometimes standing around in choruses where everybody sings.
Odom and company took the Handbook of the Birds of the World, a 16-volume set printed in Spain, covering every known living species of birds. Every section contained what ornithologists knew about that species. They also took other publications that described bird species.
In an exhaustive review of the texts, Odom and her colleagues found 323 species in which the females was either known to be able to sing or specifically not. To the surprise of most scientists, in 71 percent of these species, the females could sing, far more than anyone expected.
Then they used a phylogenetic tree, which follows genetic attributes of organisms to trace the evolution of the birds back to their common ancestors, and discovered that the common ancestor of the songbirds had female singers.
“No, it’s not rare and it’s not some recent anomaly, and is probably how songs first evolved. It turns the entire field of bird song on its head and that was really, really surprising,” Omland says. The results were not expected, he continues, but the conclusion was much clearer than they thought they would find.
Those conclusions were also a surprise to fellow ornithologists.
“That’s not anything we realized until the folks at [UMBC] did their work,” Cornell’s Webster says. “Both males and females sang. If you asked somebody a year ago about the ancestral songbird they’d say the males sang and the females didn’t. Now we know.”
Now Omland and Odom and other UMBC researchers are turning to the questions that aren’t answered by their pioneering work.
“One of the things not emphasized in our study is that for the majority of species, we still don’t know if females sing,” Omland said.
They are working on it.
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