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Bergmann’s Rule May Hint at Adapability of Song Sparrows

Two photos of Song Sparrows showing differences in subspecies.
Subspecies of Song Sparrows can be wildly different in size: The maxima subspecies, photographed in the West Aleutian Islands, can weigh twice as much as subspecies merrilli, photographed in Washington State. Photos from Macaulay Library by Julio Mulero and Steven Mlodinow.

From the Spring 2024 issue of Living Bird magazine. Subscribe now.

North American Song Sparrows may have some resilience to climate change built into their genes, thanks to a remarkable adapta­tion that accounts for the stunning range of body sizes found throughout the bird’s westernmost range.

That adaptation was the focus of a study, published November 7 in the journal Nature Communications, that offers support for Bergmann’s Rule—a pat­tern in zoology where, broadly speak­ing, natural selection in colder climates leads to larger-bodied organisms, while warmer climates lead to smaller bodies. Among organisms that regulate their own heat, larger bodies are more effi­cient at retaining heat, while smaller bodies allow an organism to stay cooler.

The study found that Song Sparrows that live year-round on Alaska’s Aleu­tian Islands can be up to three times larger than their cousins near San Fran­cisco Bay.

“The size difference among Song Sparrows is wild to even think about,” says coauthor Jennifer Walsh, a research associate at the Cornell Lab of Orni­thology. “Our results show that Song Sparrows have substantial capacity for adapting to local environmental change, and the genetic mechanisms underlying those changes are quite clear.”

Walsh and her collaborators con­ducted whole-genome sequencing and compared 79 genomes from nine Song Sparrow subspecies that occur along the Pacific Coast from California all the way up to the outer reaches of Alas­ka’s islands in the Bering Sea. The genome-sequencing research was con­ducted at the Cornell Lab’s Fuller Evo­lutionary Biology Program.

Map of the Northwest of North America with four illustrations of a species of Song Sparrow showing the difference in size compared to where the subspecies lives.
The Largest Song Sparrows Live in the North. Song Sparrows in the Aleutian Islands can be twice as big as the Song Sparrows in British Columbia (and three times bigger than the same species in the San Francisco Bay Area). Colors indicate ranges of some Song Sparrow subspecies: yellow = Melospiza melodia maxima, body weight 46.9 g; red = M. m. sanaka, 44.4 g; green = M. m. rufina, 29.2 g; purple = M. m. merrilli, 23.4 g. Map graphic from Carbeck et al. 2023. Illustrations by Jillian Ditner.

“We found eight gene variations in the genomes we sequenced, all asso­ciated with body mass as predicted by Bergmann’s Rule,” says lead author Katherine Carbeck, a PhD candidate at the University of British Columbia in Vancouver. “What this tells us is that there is a genetic basis for Song Sparrow adaptation to local climate conditions, stretching from the coldest locations in the far north to the warmest parts of its range in California.”

Understanding the nuances of micro­evolution makes a difference when it comes to conservation, the scientists said. For example, eBird Trends maps show that Song Sparrow populations in northwestern regions, such as Alaska and British Columbia, are stable or increasing currently, but sparrow popu­lations in California are declining.

While declines in one portion of the Song Sparrow’s range could mean loss of genetic diversity in locally specialized populations, Peter Arcese, a coauthor on the study and profes­sor in UBC’s Department of Forest and Conservation Sciences, says the find­ings suggest a resilient future for these birds—as long as they have habitat.

“Our findings imply that some, if not all, locally adapted Song Sparrow popu­lations may continue to adapt to climate change, as long as we maintain habitat conditions that facilitate the movement of individuals and genes between popu­lations,” he says.

“These genomic discoveries show that Song Sparrows have been waxing and waning over millennia. Over that time, each sparrow population has become beautifully fine-tuned to its local environment,” adds Irby Lovette, another coauthor on the study and director of the Fuller Evolutionary Biol­ogy Program at the Cornell Lab. “So the valid concern is that as environments change we might lose those tightly matched adaptations.

“Yet the good news is that this very same genetic mosaic creates the raw material for the sparrows of the future to adapt more quickly, just as long as their populations remain healthy over­all such that they can move around to match their traits to changing local con­ditions.”

This study is the first result of a larger Cornell Lab research effort to sequence Song Sparrow genomes from across North America, spanning nearly all of the 25 recognized subspecies.

The Cornell Lab

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