The Bird Species That Sing the Same Tune for Hundreds of Thousands of Years – Technology Networks

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Many of the birds that awaken us each morning learn their melodious songs the same way that humans learn a dialect — from parents and neighbors.

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

A recent study by biologists from the University of California, Berkeley, and Missouri State University in Springfield, however, documents songs in East African sunbirds that have remained nearly unchanged for more than 500,000 years, and perhaps for as long as 1 million years, making the songs nearly indistinguishable from those of relatives from which they’ve long been separated.

The amazingly static nature of their songs may be due to a lack of change in these birds’ environments, which are stable mountain forests — so-called sky islands — isolated from other sky island populations of the same or similar species for tens of thousands to millions of years. The coloration of the birds’ feathers has changed little, as well, making their plumage nearly indistinguishable from each other, even though some are separate, but closely related, species.

“If you isolate humans, their dialects quite often change; you can tell after a while where somebody comes from. And song has been interpreted in that same way,” said senior author Rauri Bowie, UC Berkeley professor of integrative biology and a curator in the campus’s Museum of Vertebrate Zoology. “What our paper shows is that it’s not necessarily the case for birds. Even in traits that should be very labile, such as song or plumage, you can have long periods of stasis.”

Bowie said that the idea that bird song readily changes likely arose from studies of Northern Hemisphere birds, which have encountered changing environmental conditions repeatedly, with glaciers coming and going over the last tens of thousands of years. Changing environments cause changes in plumage, bird song, mating behavior and much more.

But mountaintop environments in the tropics, particularly in Eastern Africa — from Mt. Kenya to Mt. Kilimanjaro in Tanzania south through Malawi into Mozambique — have seen little geological change over that same time period. Hence, the birds the researchers studied — lineages of the eastern double-collared sunbirds in the genus Cinnyris — had no impetus to alter either their colorful plumage or their often intricate songs.

“For social signals like song, or bright plumage colors, to take another example in birds, evolutionary biologists have focused a great deal on how the signals seem to be able to evolve quickly and in pretty random directions,” said first author Jay McEntee, an assistant professor of biology at Missouri State. “The result from our work — that there can be long periods without much change for learned song — suggests we should be asking which forces can constrain these social signals over time, in addition to asking which forces cause changes.”

According to Bowie, biologists recognize two types of barriers to mating that tend to generate new species of animals. Pre-mating barriers are signals that stop an individual from mating with another. In birds, this could be because it doesn’t sing the right song or doesn’t look the same as the other bird, but if the two did mate, they could produce offspring. Post-mating barriers are actual mechanistic reproductive incompatibilities, such that they don’t produce offspring, even if they mate.

“Song is thought to be one of the most important pre-mating isolation barriers, one of the key ways that birds tell each other apart,” he said. “That a learned trait can remain static for hundreds to thousands of years is simply remarkable, a discovery that reflects how much the field study of tropical systems has to offer the scientific community and curious observer.”

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

Reference: McEntee JP, Zhelezov G, Werema C, et al. Punctuated evolution in the learned songs of African sunbirds. Proc. Royal Soc. B. 2021;288(1963):20212062. doi: 10.1098/rspb.2021.2062

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