Many songbirds learn their tunes by listening to family members and neighbors. They mimic what they hear, so the songs change a little here and there over the years as they replicate the sounds.
But a new study finds that songs of East African sunbirds have remained almost unchanged for more than 500,000 years. They may even have stayed the same for as long as 1 million years, making their tunes nearly the same as long-lost relatives.
For their research, scientists studied eastern double-collared sunbirds (Cinnyris mediocris), which live in the mountains of the Eastern Afromontane, mountain ranges that make up a biodiversity hotspot in East Africa. Sunbirds are brightly colored birds that live mostly on nectar. They are known for complex, territorial songs that are strikingly different from other species.
Biologists from the University of California, Berkeley, and Missouri State University in Springfield, worked on the study.
“We were interested in speciation (how new species arise), and in particular how traits diverge among populations during the speciation process. Isolation is thought to be important in this process in birds,” first author Jay McEntee, an assistant professor of biology at Missouri State University, tells Treehugger.
“The sky island forests in Eastern Africa end up being a good place to study populations that are isolated from one another.”
These “sky island sunbirds” are sunbird species that live apart from other birds on the tops of high mountains in forests known as sky islands.
Senior author Rauri Bowie, UC Berkeley professor of integrative biology, did his Ph.D. thesis on these birds, demonstrating that what people once thought were two species of eastern double-collared sunbirds that were found over many East Africa mountaintops were actually five or maybe six species. They looked alike, but were different genetically, making Bowie wonder if their songs had remained unchanged, just like their feathers.
McEntee teamed up with Bowie to find out, visiting nearly all of the sky islands in East Africa to record 356 songs from 123 different birds from the six lineages of eastern double-collared sunbirds.
“Carrying out this research was just a great experience. We met many wonderful people while traveling around to these different sky islands. And yes, there were times when we went to make sound recordings of populations, and the birds sounded nothing like what we imagined they would, or they were obviously different than what we expected in some particular respect,” McIntee says.
“Other times, especially when Rauri’s work had shown a deep genetic difference for a particular population, we expected to find birds that sang differently from their closest relatives, and they just didn’t. This was a little deflating at times, but because it was pretty surprising, it ended up as a really interesting part of the story.”
Their findings were published in the journal Proceedings of the Royal Society B Biological Sciences.
Learning Why Things Change
Birds typically learn their songs from their parents and nearby birds. But this learning process is prone to errors and change over time.
“Many song-learning birds construct their songs based on what they hear from other birds of their own species. However, an individual’s songs are not direct copies of what they’ve heard other birds sing. Birds mix components of different songs they’ve heard, and add variation that is a bit like improvisation,” McIntee says.
“In this way, just as languages evolve by these processes, bird song can evolve by these processes. It’s been expected that these kinds of changes add up in isolated populations, over time, and that should cause divergence in measurements we can make like song duration or pitch.”
Researchers don’t know for sure why songs haven’t evolved over time with the East African sunbirds.
“One thing that’s really interesting about the sky islands where these sunbirds live is that they seem to have a high level of environmental constancy over time. Relative to other places, these sky islands seem to have had a fairly constant climate, and there is evidence that forest has consistently covered them during global climate changes that caused radical changes in ecological communities elsewhere. We think that’s an important factor,” McIntee says.
“There are a number of species that live across sky islands in East Africa that have changed very little in other traits (plumage, morphology) despite extended periods of isolation.”
Learning why things change—and why they don’t—is important for science, the researcher says.
“We spend a lot of time in evolution trying to understand why things change,” says McIntee. “It’s not a novel point to make that we need to spend time thinking about what constrains change too, but I think that we’ve found this kind of constancy in learned traits, of all things, over time, really underscores this point.”
