Tiny Wrens Turn Giant on Scottish Isles

A mainland wren weighs about as much as a folded piece of paper: 7 to 10 grams, round and brown and perpetually outraged-looking. Wrens inhabiting remote Scottish islands have evolved to reach twice the mass of their mainland relatives. Which raises a reasonable question: what, exactly, is happening out there?

Research in the Evolutionary Journal of the Linnean Society finds that Scotland's island wren populations are each on separate evolutionary trajectories across four locations: Shetland, Fair Isle, the Outer Hebrides and St Kilda. The island populations are cut off from each other but share comparable habitats, all diverging markedly from mainland British and European wrens.

This is "island gigantism": a biological phenomenon in which an island-isolated animal grows dramatically larger than its mainland relatives. The wrens of Shetland and St Kilda are the standout cases. At their largest, St Kilda wrens run more than double the size of their smallest mainland counterparts, a gap that ranks among the top quarter of island gigantism cases in birds worldwide.

The more unsettling finding is what the genetics suggest. Lead author Dr. Michał Jezierski found that all four Scottish wren subspecies are genetically distinct from mainland British wrens, with Shetland and St Kilda birds especially distinct in both appearance and song. Their genetic divergence is significant enough that they are likely on their way to becoming new species. Beyond size, the birds have also developed unique vocalizations, altered feather patterns and modified body structures.

Same archipelago, same winds, same general misery of Scottish weather. Different evolutionary outcomes. Why changes in body size and other island traits represent adaptations to specific ecological conditions remains, for now, a mystery.

The wrens of St Kilda have apparently decided not to wait for us to figure it out.

Read the full story at University of Birmingham, May 28, 2026

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