Remote Scottish islands turned tiny wrens into giant birds that may be evolving into new species.
Tiny wrens living on remote Scottish islands are providing scientists with a rare look at how evolution can reshape animals in isolated environments. A new study led by researchers at the University of Birmingham found that several island populations of wrens have followed their own evolutionary paths, with some growing dramatically larger than their mainland relatives.
The findings, published in the Evolutionary Journal of the Linnean Society, offer fresh insight into the biological phenomenon known as “island syndromes.” These are recurring evolutionary patterns seen in species that become isolated on islands.
Researchers focused on four subspecies of wrens found only on Scottish islands and archipelagos: Shetland, Fair Isle, the Outer Hebrides, and St Kilda. Although these birds live in broadly similar island environments, each population has remained geographically separated and differs noticeably from wrens found across mainland Great Britain and continental Europe.
Giant Wrens on Remote Scottish Islands
One of the most striking discoveries involved a phenomenon known as island gigantism, in which animals evolve larger body sizes after becoming isolated on islands.
Famous examples include the giant tortoises of the Galápagos Islands and the extinct dodo of Mauritius, both of which became much larger than their mainland ancestors. The new study shows that Scottish wrens may represent an unusually extreme bird example of the same process.
Scientists found that wren populations on Shetland and St Kilda show very little evidence of interbreeding with mainland birds. These isolated populations have also grown significantly larger over time.
A typical wren from England weighs between 7 and 10 grams. By comparison, wrens living on St Kilda weigh between 13 and 16 grams. The largest St Kilda birds are more than twice the size of the smallest wrens found on mainland Great Britain. According to the researchers, this places them among the top 25% of known cases of island gigantism in birds worldwide.
Dr. Michał Jezierski, from the School of Geography, Earth and Environmental Sciences and lead author of the study, said: “We found that all four Scottish Wren subspecies are genetically distinct from the Wrens of mainland Britain; with the Wrens of Shetland and St Kilda being especially distinct in both appearance and song. Their genetic distinctiveness is so high, that it is likely they are on their way to becoming new species.”
Signs of New Species Emerging
To investigate these populations, researchers combined body measurements, song recordings, and whole genome sequencing. This approach allowed them to compare island and mainland wrens in unprecedented detail and better understand how island syndromes develop.
The results showed that each island population has become genetically distinct and remains largely isolated from the others.
The wrens of Shetland and St Kilda are particularly interesting because they appear very similar physically, yet the genetic changes that separate them from mainland wrens are largely different. In other words, the two groups arrived at similar outcomes through different genetic routes.
By contrast, wrens living on Fair Isle and in the Outer Hebrides remain more similar to mainland populations. This suggests that even neighboring island groups do not necessarily evolve in the same way.
Parallel Evolution in Action
According to the researchers, the similarities between the Shetland and St Kilda wrens are an example of parallel evolution.
Dr. Jezierski explained: “Our genomic data indicates that Shetland and St Kilda Wrens are genetically distinct from each other, despite their similarities in physical appearance. This means that their island gigantism is a case of ‘parallel evolution’, where a similar original population (probably colonists from the British mainland) made it to each island archipelago, and then independently evolved to become island giants. In the process, their songs also became very different from those of ‘mainland’ British birds.”
The findings suggest that similar island environments can push populations toward comparable traits, even when the underlying genetic changes differ.
Will Smith, from the University of Nottingham and a co-author of the study, said: “Our research suggests that islands with similar environments can produce similar evolutionary outcomes using different genetic pathways. The Wrens of Scotland provide us with a powerful case study to understand the mechanisms by which island biodiversity is generated worldwide.”
Unlocking the Mystery of Island Evolution
Islands are home to an estimated 20% to 30% of the world’s species and are known for producing unusual forms of wildlife, from Madagascar’s lemurs to Indonesia’s Komodo dragons. Because islands are naturally isolated and often have fewer predators and competitors than nearby mainland regions, they create unique evolutionary conditions.
Scientists have observed island syndromes in a wide range of plants and animals around the world. Common traits include larger body size, longer lifespans, slower reproduction, and in birds, reduced flight ability. Despite how widespread these patterns are, researchers still do not fully understand the biological mechanisms that drive them.
The Scottish wrens also show other traits commonly linked to island evolution. Along with their larger size, they have developed distinctive songs as well as subtle differences in plumage and body shape.
Although the reasons behind island gigantism and other island syndromes remain uncertain, the researchers say these wren populations provide an exceptional opportunity to explore how evolution works in isolated environments. By studying these birds, scientists hope to better understand the small-scale evolutionary processes that eventually produce the remarkable biodiversity seen on islands around the world.
Reference: “Parallel evolution of island syndromes coincides with limited parallel genetic differentiation in a passerine bird” by Michał T Jezierski, Jenny C Dunn, Carolina R F Chagas and William J Smith, 28 May 2026, Evolutionary Journal of the Linnean Society.
DOI: 10.1093/evolinnean/kzag008
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