Bird species that have been singing the same tune for hundreds of thousands of years


Many of the birds that wake us up every morning learn their melodious songs the same way humans learn a dialect – from relatives and neighbors.

But for most biologists, learning songs through mimicry is an uncertain and error-prone process, resulting in a slow but inevitable change in song over the years.

A recent study by biologists from the University of California, Berkeley and Missouri State University, Springfield, however, documents the songs of East African sunbirds that have remained virtually unchanged for over 500,000 years. years, and possibly as long as a million years, making the songs almost indistinguishable from those of relatives from whom they have long been separated.

The surprisingly static nature of their songs may be due to the lack of change in the environment of these birds, which are stable mountain forests – called sky islands – isolated from other populations of sky islands of the same species or d similar species for tens of thousands of years. millions of years. The coloration of the birds’ feathers has also changed little, making their plumage nearly indistinguishable from each other, even though some are distinct, but closely related, species.

“If you isolate humans, their dialects change quite often, after a while you can tell where someone is from. And the song was performed the same way,” the lead writer said. Rauri Bowie, professor of integrative biology at UC Berkeley and curator of the campus Museum of Vertebrate Zoology. “What our paper shows is that this is not necessarily the case for birds. Even in traits that should be very labile, like song or plumage, you can have long periods of stasis.”

Bowie said the idea that birdsong changes easily probably arose from studies of Northern Hemisphere birds, which repeatedly encountered changing environmental conditions, with glaciers coming and going over the past tens of thousands of years. Changing environments lead to changes in plumage, bird song, mating behavior and much more.

But mountainous environments in the tropics, particularly in East Africa – from Mount Kenya to Mount Kilimanjaro in Tanzania to southern Malawi in Mozambique – have seen little geological change over the same period. Therefore, the birds the researchers studied – the lineages of the eastern double-collared sunbirds of the genus Cinnyris – had no desire to modify either their colorful plumage or their often complex songs.

“For social cues like song or bright plumage colors, to take another example in birds, evolutionary biologists have focused a lot on how cues seem to be able to evolve quickly and in fairly random directions. “said first author Jay McEntee, an assistant professor of biology at Missouri State. “The result of our work – that there can be long periods of little change for the learned song – suggests that we should ask what forces can constrain these social cues over time, in addition to asking what forces cause changes.”

According to Bowie, biologists recognize two types of mating barriers that tend to generate new species of animals. Pre-mating barriers are signals that prevent one individual from mating with another. In birds, this could be because it sings the wrong song or doesn’t sound like the other bird, but if the two mate, they could produce offspring. Post-mating barriers are true mechanistic reproductive incompatibilities such that they do not produce offspring even if they mate.

“Singing is thought to be one of the most important isolation barriers before mating, one of the main ways birds distinguish themselves,” he said. “That a learned trait can remain static for hundreds or thousands of years is nothing short of remarkable, a discovery that reflects all that the field study of tropical systems has to offer the scientific community and the world. curious observer.

McEntee, Bowie and their colleagues in Africa, Europe and the United States published their findings last month in the journal Proceedings of the Royal Society B.

Sky Island Sunbirds

Sunbirds are the most diverse and some of the most colorful groups of birds in Africa and Asia, filling a nectar-eating niche occupied by the hummingbirds of the Americas, although they are not related to the hummingbirds. Many have curved beaks that allow them to dive deep into the curved flowers. Bowie calls them “the little gems that appear in front of you.”

Among these wide-ranging birds are species that have become isolated on the tops of high mountains, hence the term “sky island simanga”. Bowie, from South Africa, has long been intrigued by these birds – for his doctorate. thesis, he showed that what people once thought were two species of eastern double-collared sunbird distributed over many mountain peaks in East Africa were actually at least five species and possibly six. Although they look very similar, they are genetically deeply divergent, suggesting a long period of isolation on different mountain peaks where they are now recognized as different species.

He began to wonder, however, if their songs had remained as unchanged as their plumage. McEntee pursued this question as a doctoral student with Bowie, visiting 15 – almost all – of the sky islands of East Africa between 2007 and 2011 and recording songs from 123 individual birds from six different lineages of double-collared sunbirds. ballast. The sky island populations of these sunbirds are ideal for exploring the evolution of song, since the isolated populations allowed comparison within individual species, between two closely related species, and between all five species.

The researchers developed a statistical technique to test between gradual change and bursts of rapid change in a trait such as bird song and found that song differences did not appear to be correlated with the length of time individual populations were separated, as estimated from genetic differences in their DNA. Two populations of long-separated species had nearly identical songs, while two other similar-looking species that had been separated for less time had widely divergent songs.

“What surprised me most about doing this research was how similar these songs learned from isolated populations were, within species, and how obvious the song differences were where they were. produce,” McEntee said. “The first time (Tanzanian co-writer) Maneno Mbilinyi and I made a sound recording of Cinnyris fuelleborni, what we call the Fueleborn sunbird, we figured there must be a different bird nearby that was singing simultaneously, because the song we were listening to didn’t make sense to us. We looked directly at the songbird, watching it move its beak, and we couldn’t believe how different its song was from Moreau’s sunbird, which sounded very similar, Cinnyris moreaui, which we had just recorded in another part of the Udzungwa Mountains.”

On the other hand, the songs of Cinnyris fuelleborni populations from Ikokoto in Tanzania and Namuli in Mozambique are almost identical, although they are separated by hundreds of kilometers and hundreds of thousands of years.

Based on this study, biologists argue that features such as learned song and plumage do not inevitably drift in isolated populations, but evolve in pulses, punctuated by long periods, perhaps hundreds of thousands years, little change.

“We show, using a really nice setup where we could look at the evolution of songs using naturally isolated populations, that you don’t see this gradual change by cultural or genetic drift at all,” Bowie said. “You see these abrupt changes in a trait like a bird song and lots of evidence of stasis, even when that trait should be very plastic. For me, that was a really fascinating result.”

Researchers are continuing their research in East Africa to determine what causes some birds to change their songs, but not others. Bowie and UC Berkeley professors Jim McGuire and Robert Dudley are part of a $2 million grant from the National Science Foundation () to understand the genetic and physiological causes of convergent evolution that make sunbirds and hummingbirds so similar in appearance and ecological niche.

Reference: McEntee JP, Zhelezov G, Werema C, et al. Punctuated evolution of the scholarly songs of African sunbirds. proc. Royal Society B. 2021;288(1963):20212062. do I: 10.1098/rspb.2021.2062

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