A new study of British Wrens has provided new insights into the inner workings of ‘island syndromes’, according to research led by the University of Birmingham.
The paper, published in the Evolutionary Journal of the Linnean Society , reveals that different subspecies of island Wrens are evolving independently, with the team finding particularly strong evidence of ‘island gigantism’ in two of the studied populations.
Researchers examined four subspecies of island Wrens, each found on a specific island or archipelago in Scotland – Shetland, Fair Isle, the Outer Hebrides, and St Kilda. Each of these subspecies is geographically isolated (but exposed to broadly similar environments on different Scottish islands), and each differs significantly from the Wren subspecies found across mainland Great Britain and continental Europe.
Island gigantism is a biological phenomenon in which the size of an animal species isolated on an island increases dramatically in comparison to its mainland relative. This is most famously exemplified by the giant tortoises of the Galapagos, and the extinct Dodo of Mauritius; both of which far exceed the sizes of their continental ancestors.
Led by Dr Michał Jezierski, the study’s findings represent one of the most in-depth explorations of the population-level processes giving rise to island syndromes. These evolutionary phenomena, detected across disparate types of animals and plants across Earth’s islands, involve a suite of evolutionary changes in island species including; island gigantism, longer lifespans, slower rate of reproduction and, in birds, a tendency towards lower flight ability.
Key findings from the study include:
Wren populations on Shetland and St Kilda show minimal evidence of interbreeding with their cousins from mainland Britain.
These two subspecies have evolved spectacular island gigantism; a Wren from England will weigh 7-10g on average, whereas on St Kilda they range from 13-16g.
The largest St Kilda Wrens are more than twice the size of the smallest on mainland Great Britain, putting them within the top 25% of 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.”
By comparing these four Scottish island subspecies with mainland Wrens, using body measurements, song recordings, and whole genome sequencing, researchers were able to explore the differences in biology between the different island populations in more detail than has previously been possible, allowing them to further investigate the ways in which island syndromes evolve.
Islands host 20-30% of species worldwide and are famous for their unusual wildlife – from Madagascan lemurs to Komodo Dragons. Similar conditions found across the world’s islands, including lower predation and competition than on adjacent mainlands, are driven by their inherent isolation. Although island syndromes may be found across much of our planet’s biodiversity, their underlying mechanisms are poorly understood.
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.”
Shared traits, different genetics
Whole genome comparisons showed that each island population is genetically distinct and largely isolated: while the Wrens of Shetland and St Kilda are physically similar, the regions of their genomes that show the most differences from mainland Wrens are largely independent from each other.
By contrast, Wrens from Fair Isle and the Outer Hebrides are more similar to those on the British mainland: highlighting that island evolution does not proceed in the same way, even within a relatively small geographic region, with each population representing a related, but largely independent, evolutionary unit.
Dr Jezierski added: “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.”
Understanding the ‘micro’ processes that lead to ‘macro’ patterns
The ‘giant’ size of the Wrens has evolved alongside other island associated traits, including distinctive songs and subtle differences in plumage and body proportions, supporting the idea that island environments consistently shape evolution in predictable directions.
The ‘why’ of island syndromes remains a mystery, and researchers do not yet fully know how changes in body size, or other island syndromes, represent adaptations to the special ecological conditions on islands. However, these latest findings amongst Wren populations may provide a great case study with which Birmingham researchers can further investigate this field of evolutionary science.
Evolutionary Journal of the Linnean Society
Observational study
Animals
Parallel evolution of island syndromes coincides with limited parallel genetic differentiation in a passerine bird
28-May-2026