They are tiny birds, but on some Scottish islands they are getting very big.
A study led by the University of Birmingham found several isolated island populations of wrens are evolving independently, with some growing much larger than their mainland relatives.
The research, published in the Evolutionary Journal of the Linnean Society, examined four island subspecies in Scotland: Shetland, Fair Isle, the Outer Hebrides and St Kilda.
Researchers found especially strong examples of island gigantism in wrens from Shetland and St Kilda. Mainland British wrens typically weigh between 7 and 10 grams. On St Kilda, the birds weighed between 13 and 16 grams.
According to the researchers, the largest St Kilda wrens are more than twice the size of the smallest wrens found on mainland Great Britain. They said that places them among the top 25 percent of known cases of island gigantism in birds around the globe.
Scientists also found little evidence that wrens from Shetland and St Kilda regularly interbreed with mainland populations. The study says their long isolation has allowed them to develop distinct physical traits, songs and genetics.
Lead author Dr MichaÅ‚ Jezierski, from the University of Birmingham’s School of Geography, Earth and Environmental Sciences, 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.”
To study the birds, researchers compared island wrens with wrens from mainland Britain using body measurements, song recordings and whole genome sequencing.
Genome analysis showed each island population is genetically distinct and largely isolated from the others. Although wrens from Shetland and St Kilda look similar, the genetic changes linked to their evolution are mostly different.
Researchers said that points to parallel evolution, where similar environmental pressures produce similar outcomes independently.
Dr Jezierski said: “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.”
Wrens from Fair Isle and the Outer Hebrides remained more genetically similar to mainland birds, showing island evolution can vary significantly even across nearby islands.
Co-author Will Smith, from the University of Nottingham, 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.”
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