Songbirds that undertake the challenging migration from their breeding grounds in northern boreal forests to warmer southern regions during winter may gain a reward of increased genetic variation, as indicated by a study from the University of Michigan.

Researchers at U-M discovered that songbird species migrating from the boreal forests of the northern United States and Canada to tropical areas for winter exhibit greater genetic diversity compared to those that remain close to their northern breeding grounds, either by not migrating or only traveling short distances, such as to the southern United States.

The scientists also found that long-distance migratory birds are less inclined to experience extensive gene flow, meaning the movement of genetic material between distinct populations. Their findings, which underscore the relationship between genetic diversity and the impacts of climate change and human influence on avian habitats, are published in the journal Nature Ecology & Evolution.

High genetic diversity signifies a population with a wide range of gene variants, both within individual organisms and across the species population. Genetic diversity can narrate a species’ evolutionary journey: it reveals the extent of genetic variation that has been conserved versus lost due to historical fluctuations in population size.

“Long-distance migration has developed into a highly effective strategy for birds, because despite its challenges and requiring exceptional physiology, it enables them to spend the northern winter in areas where they have higher survival rates,” explained 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 findings imply is that these exceptionally long-distance migrants have likely maintained more stable populations throughout evolutionary history, which is remarkable considering they undergo these extraordinary, perilous migrations.”

Migration, genetic diversity, and gene flow

According to the researchers, scientists have pondered the impact of migration on bird population genetics for an extended period. However, the financial barrier posed by genetic sequencing technology—at least at the scale necessary to execute this type of comparison among genomes that are likely very similar—was a significant impediment.

Nonetheless, Winger and collaborators began sampling bird populations within the boreal forest for genetic analysis over 15 years ago, aiming to eventually complete the study. They also utilized genetic samples from various museum collections, including the UM Museum of Zoology.

Many of the birds share close relationships but engage in different migration behaviors, noted Winger. For instance, hermit thrushes and Swainson’s thrushes are closely allied species that breed in the same boreal area. However, the hermit thrush winters further north in the southern United States, while the Swainson’s thrush migrates to South America.

The researchers initially investigated the birds’ genomes to assess the effects of long-distance migration on gene flow. This posed a challenge, stated first author Teresa Pegan, who completed the study as a doctoral student at U-M, since the populations within each bird species across the boreal forest were so closely related. All of them inhabited areas within the boreal forest, the predominantly coniferous forest encircling the globe in the northern hemisphere.

“We specifically structured this study to explore this extensive area that operates as a single population, signifying no barriers within it,” remarked Pegan, now a postdoctoral researcher at Harvard University. “If a bird hatches in Ontario, there’s not much stopping it from traveling to Manitoba if it chooses to.”

This suggests that bird populations might be interfacing, blending their genomes and obscuring any genetic distinctions that could set them apart. The researchers anticipated that any patterns would be subtle.

To identify these subtle variations, Pegan sequenced 1,700 genomes from 35 different species of birds. She then created a computational analysis allowing the team to detect genetic patterns within each species. These genetic patterns would enable the researchers to establish the gene flow of the birds, referring to the movement of genetic variants between populations in the boreal forest.

If migratory birds frequently traversed the continent to breed and mated with other groups of the same species, the researchers would not expect to observe genetic patterns that align with geographic context, or the specific breeding locations of the birds. Distinct genetic patterns would be unlikely.

However, in certain long-distance migratory species, the researchers uncovered correlations between genetic patterns and breeding locations within the boreal forest. These correlations manifest when two birds from the same region are statistically more likely to be related to one another than to a bird located hundreds of kilometers away. These distinct patterns indicated that birds that were closely related to each other tend to remain spatially close over evolutionary time, Pegan noted. In essence, migratory birds consistently return to the same breeding grounds each year, even though they undertake extensive migrations in the winter.

The researchers also investigated migration’s influence on genetic diversity, not anticipating a significant impact.

“But to our astonishment—I nearly fell from my chair when I discovered it—there exists a remarkably strong correlation between migration distance and the level of genetic diversity within the population,” said Pegan.

“This influence is so substantial that a strong correlation can even be observed between individual birds from different species. For instance, if you take an individual from a species that migrates a greater distance, in this particular comparison, that bird exhibits more diversity within its own genome than an individual that either does not migrate or migrates shorter distances.”

Human impact and climate change

The researchers assert that their findings can offer critical insights for conservation initiatives aimed at helping these birds withstand the pressures of climate change and human effects on their habitats. For instance, their results indicate that certain bird groups return to a small, specific area of forest each year to breed.

“The boreal forest across Canada and the United States faces significant pressures, not only from climate change but also from resource extraction,” Winger stated. “Deforestation in the boreal forests throughout the U.S. and Canada is more severe than many individuals realize.”

“Our research shows that some species likely experience considerable gene flow, suggesting they move extensively throughout their breeding range. However, others return to the same forest patch annually, making them potentially more vulnerable to deforestation. They may struggle to travel 10 miles away if they find their nesting area has been cleared.”

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 post-embargo)