Songbirds with longer migrations have greater genetic diversity

Birds that embark on long, perilous journeys from the northern boreal forests to tropical destinations each winter gain more than just a warm escape. These songbirds may also carry a hidden advantage – greater genetic diversity.

This insight comes from researchers at the University of Michigan. The team compared the genomes of 35 songbird species that nest in boreal forests across the United States and Canada.

The study revealed that species migrating longer distances to overwinter in tropical regions had more genetic diversity than species staying closer to home.

These birds also showed distinct genetic patterns linked to their breeding sites, suggesting a faithful return to the same nesting grounds year after year.

Songbird migration and genetic variation

The researchers found a strong connection between how far birds migrate and the level of genetic variation they carry.

Genetic diversity reflects how many different versions of genes are present within a population. A higher level of diversity often points to a more stable population over evolutionary time.

“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 Benjamin Winger, lead author of the study.

“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.”

This stability in the face of extreme effort was unexpected. Researchers had long wondered about the relationship between migration and bird genetics, but until recently, analyzing large sets of bird genomes wasn’t feasible due to cost and technical limitations.

Gene flow and songbird migration

The research team started collecting samples over 15 years ago. They tapped into multiple museum collections – including the University of Michigan’s Museum of Zoology – to gather enough data for their analysis.

Many of the songbird species were closely related, with different migration behaviors, offering a unique way to isolate the impact of distance traveled.

Take the hermit thrush and the Swainson’s thrush. These birds breed in the same boreal regions. But while the hermit thrush winters in the southern U.S., the Swainson’s thrush ventures all the way to South America.

The goal was to understand gene flow – the transfer of genetic material between populations – and how it’s affected by migration.

Strong ties to breeding sites

In theory, if birds frequently move across the continent and breed with distant groups, their genes should be well-mixed. That would make it harder to detect distinct genetic patterns tied to breeding locations. But the study revealed something different.

“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 Teresa Pegan, 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.”

Still, the genetic analysis told a different story. Pegan sequenced 1,700 genomes and developed a computational method to identify genetic patterns within each species.

Some long-distance migrants, it turns out, showed clear genetic ties to specific breeding sites – evidence that they return to the same location each year, despite flying thousands of miles.

An unexpected genetic surprise

Initially, the team wasn’t expecting songbird migrations to strongly affect genetic diversity. But the data proved otherwise.

“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,” said Pegan.

“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.”

This means that species traveling farther tend to carry more genetic tools to adapt and survive – an evolutionary advantage in a world of changing environments.

Protecting the right places

The findings carry important implications for conservation. If long-distance migrants are faithful to specific breeding sites, then those areas are critical to protect – especially as forests face threats from logging and development.

“The boreal forest throughout Canada and the United States experiences substantial pressure, not just from climate change, but from resource extraction,” said Winger. “Deforestation of boreal forest throughout the US and Canada is more severe than many people realize.”

Some bird populations seem flexible, moving freely across breeding territories. But others return to the same forest patch every year. These site-loyal birds are especially vulnerable if their nesting areas are destroyed.

“They may be less able to fly 10 miles away if they come back and their nesting site has been clear cut,” Winger explained.

Understanding how birds migrate and maintain their genetic health helps scientists make smarter decisions about which habitats to preserve and how to manage growing environmental stressors.

The research highlights that the incredible journeys of migratory birds aren’t just feats of endurance – they’re deeply tied to the survival and resilience of their species.

The full study was published in the journal Nature Ecology & Evolution.

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