Wrens inhabiting remote Scottish islands are undergoing a remarkable evolutionary transformation, developing 'gigantism' due to the absence of predators and competition. These birds have grown up to twice the size of their mainland counterparts, according to new research.
Island Gigantism in Wrens
In England, wrens typically weigh between seven and ten grams. However, on the St Kilda archipelago, they range from 13 to 16 grams—more than double the mass of mainland birds. Shetland wrens are also larger, weighing approximately 2.9 grams more than British mainland wrens, though not as massive as those on St Kilda. Wrens on the Outer Hebrides are nearly a gram heavier than mainlanders.
This growth, known as 'island gigantism,' occurs when animals in isolated environments are shielded from predators and competition, allowing them to thrive. Normally, wrens face threats from cats, foxes, and birds of prey such as hawks and owls, as well as squirrels and rats that attack their nests. The islands provide a haven from these dangers.
Distinctive Songs and Plumage
It is not just size that has changed. The island wrens have developed distinctive songs—akin to regional accents—along with different plumage and body proportions. Similar adaptations were observed by Charles Darwin in finches, who noted how isolated populations evolved traits specific to their environments.
Dr Michał Jezierski, a biogeographer from the University of Birmingham and lead author of the study, explained: 'Their island gigantism is a case of parallel evolution, where a similar original population—probably colonists from the British mainland—reached each island archipelago and then independently evolved to become island giants.'
Genetic Distinctiveness
The research team examined wrens on Shetland, Fair Isle, the Outer Hebrides, and St Kilda, recording their songs, weight, and measurements, and analysing their genomes to compare with mainland birds. They found that all four Scottish wren subspecies are genetically distinct from mainland wrens, with those on Shetland and St Kilda being especially unique in appearance and song.
Dr Jezierski added: 'Their genetic distinctiveness is so high that it is likely they are on their way to becoming a new species.'
Fair Isle was the only population not significantly different from mainland wrens. However, each island harboured its own genetically distinct populations, with Shetland and St Kilda showing the greatest divergence.
Global Phenomenon
Island gigantism is observed worldwide, from Komodo dragons in Indonesia to the giant weta—the world's heaviest insect—and the giant tortoises of the Galapagos. Conversely, larger animals tend to become smaller when isolated, as seen with dwarf elephants that once lived in Sicily and Malta during the Pleistocene period (2.6 million to 11,700 years ago). Even tiny humans existed, such as Homo floresiensis, who stood just 3 feet 6 inches tall and lived in Indonesia around 50,000 years ago.
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.'
The findings were published in the Evolutionary Journal of the Linnean Society.
