Songbirds who make the arduous flight from their nesting sites in northern boreal forests to warm, southern climates in the winter may be rewarded for their journey with greater genetic diversity, according to a University of Michigan study.
U-M researchers found that songbird species that migrate from the boreal forests of the northern United States and Canada to the tropics to overwinter have more genetic diversity than species who stick closer to their northern breeding locations, either not migrating or migrating only short distances, for example, to the southern United States.
The researchers also found that long distance migratory birds are less likely to have extensive gene flow, or the transfer of genetic material from one population to another. Their results, which highlight the interplay between genetic diversity and the pressures of climate change and human impact on birds' habitat, are published in the journal Nature Ecology & Evolution.
Having high genetic diversity means that a population has a large breadth of gene variants both within individuals and within a species' population. Genetic diversity can tell a story of a species' evolutionary history: It shows how much genetic variation within the species has been maintained vs. lost because of greater population size fluctuation throughout history.
"Long distance migration has evolved to be a very successful strategy for birds because even though it's difficult and requires extraordinary physiology, it allows them to spend the northern winter in a place where they enjoy a higher survival rate," said lead author Benjamin Winger, associate professor of ecology and evolutionary biology and curator of birds at the U-M Museum of Zoology.
"What our genetic diversity results likely mean is that these really long-distance migrants have had more stable populations over evolutionary history, which is amazing because they're undergoing these phenomenal, risky migrations."
Migration, genetic diversity and gene flow
Scientists have been mulling migration's impact on bird population genetics for a long time, according to the researchers. But the cost of genetic sequencing technology-at least at the scale needed to do this kind of comparison between genomes that were likely very similar-was a barrier.
Still, Winger and colleagues began sampling bird populations across the boreal forest for genetic research more than 15 years ago in hopes of eventually completing the study, and also used genetic samples from several museum collections, including the UM Museum of Zoology.
Many of the birds are closely related, but have different migration strategies, Winger says. For example, hermit thrushes and Swainson's thrushes are closely related species that breed in the same boreal region. But the hermit thrush spends the winter farther north, in the southern United States, and the Swainson's thrush spends the winter in South America.
The researchers first examined the birds' genomes to look at the impact of long-distance migration on gene flow. This was a challenge, says first author Teresa Pegan, who completed the work as a doctoral student at U-M, because across the boreal forest, the populations within each bird species were so closely related. They all lived in regions within the boreal forest, the mostly coniferous forest that encircles the globe in the northern hemisphere.
"We specifically designed this study to look at this vast area that functions as a single population, meaning that there's no barriers across it," said Pegan, now a postdoctoral researcher at Harvard University. "If a bird is born in Ontario, there's not much preventing it from moving to Manitoba if it wanted."
This means bird populations could be mingling, mixing together their genomes and blurring any genetic patterns that could make them distinct. Any patterns the researchers expected to find were going to be subtle.
To tease out these subtle differences, Pegan sequenced 1,700 genomes from the 35 species of birds. She then designed a computational analysis that allowed the researchers to search for genetic patterns within each species. These genetic patterns would help the researchers determine the birds' gene flow, or movement of genetic variants across populations in the boreal forest.
If migratory birds frequently moved across the continent to breed and mated with other groups of the same species, the researchers wouldn't expect to see genetic patterns that correlate with spatial context, or where the birds breed. They wouldn't expect to see distinct genetic patterns at all.
But instead, in some long-distance migratory species, the researchers found correlations between genetic patterns and breeding location within the boreal forest. These patterns show up when two birds from the same area are more statistically likely to be related to each other than a bird from hundreds of kilometers away. These distinct patterns showed that birds that were closely related to each other are staying spatially close to each other over evolutionary time, Pegan said. In other words, migratory birds are faithfully returning to the same breeding site each year, despite migrating very long distances for the winter.
The researchers also examined migration's impact on genetic diversity, not expecting to find an outsized influence.
"But to our surprise-I almost fell out of my chair when I found it-there's actually a really remarkably strong relationship between migration distance and the amount of genetic diversity in the population," Pegan said.
"It's such a strong influence that there's even a strong relationship between individual birds from different species. For example, if you consider an individual in a species that migrates a longer distance, then in this particular comparison, that bird has more diversity within its own genome than an individual that doesn't migrate or migrates a shorter distance."
Human impact and climate change
The researchers say their findings can help provide background information for conservation efforts to help these birds survive the pressures of climate change and human impact on their habitat. For example, their results show that some bird groups return to the same small part of forest each year to breed.
"The boreal forest throughout Canada and the United States experiences substantial pressure, not just from climate change, but from resource extraction," Winger said. "Deforestation of boreal forest throughout the US and Canada is more severe than many people realize.
"We're showing that some species probably have a lot of gene flow, so they probably have a lot of movement throughout the breeding range. But others really come back to the same patch of forest every year, and those might be more susceptible to deforestation. They may be less able to fly 10 miles away if they come back and their nesting site has been clear cut."
Co-authors include researchers from Environment and Climate Change Canada, University of Lethbridge, Royal Alberta Museum, Cleveland Museum of Natural History, Spring Island Trust, New York State Museum and Colorado State University.
Study: Long-distance seasonal migration to the tropics promotes genetic diversity but not gene flow in boreal birds (DOI: 10.1038/s41559-025-02699-3) (available when embargo lifts)
