Scottish wrens are suddenly evolving into gigantic birds, scientists reveal

Scientists have uncovered wrens inhabiting remote Scottish islands have evolved to reach twice the mass of their relatives living on the British mainland.

Researchers from the universities of Birmingham and Nottingham discovered birds dwelling on Shetland and St Kilda are experiencing a phenomenon known as island gigantism, which occurs when species become isolated from typical predatory pressures.

The study reveals these island populations are genetically distinct from mainland wrens and appear to be diverging towards becoming entirely separate species.

Beyond their increased size, the birds have also developed unique vocalisations, altered feather patterns and modified body structures.

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A typical English wren weighs between 7 and 10 grams, whereas those found on St Kilda tip the scales at 13 to 16 grams.

The heftiest St Kildan specimens exceed double the weight of the lightest mainland British wrens.

Shetland's wren population averages 2.9 grams more than birds found across mainland Britain, whilst Outer Hebridean wrens carry nearly an additional gram of body weight.

This size increase stems from the islands' relative safety from threats.

Mainland wrens face danger from domestic cats, foxes, hawks and owls, with squirrels and rats targeting their nests.

Such predators are considerably scarcer on these isolated archipelagos.

Dr Michał Jezierski, a biogeographer from the University of Birmingham, described the phenomenon as "parallel evolution", explaining ancestral birds likely arrived from the British mainland to each island group before independently developing into larger forms.

"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," he said.

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"Their genetic distinctiveness is so high that it is likely they are on their way to becoming a new species."

The research team analysed body measurements, recorded songs and sequenced genomes from populations across Shetland, Fair Isle, the Outer Hebrides and St Kilda.

DNA analysis confirmed every island population possesses distinct genetic markers, with Shetland and St Kildan birds showing the greatest divergence from mainland relatives.

Fair Isle represented the sole exception, with wrens there showing no significant size difference from British mainland populations.

The island birds also sing more slowly, incorporating buzzing sounds absent from mainland songs.

St Kildan wrens produce shorter calls with reduced frequency range, whilst Shetland birds have developed elongated, deeper bills. Their St Kildan counterparts possess broader bills alongside lengthier tails and wings.

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."

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