Wrens on St Kilda are evolving into a separate giant species due to their isolation on the archipelago, a new study has found.

As a result, the bird population now weighs significantly more than their mainland counterparts.

Island gigantism on St Kilda, which is seen in other island isolated species such as the giant tortoises on the Galápagos and Indonesia’s 140 kg Komodo Dragons, has been revealed in a new study in the Evolutionary Journal of the Linnean Society.

Researchers from Birmingham University compared wrens (Troglodytes troglodytes) on St Kilda, the Western Isles, Shetland, and Fair Isle. Adult wrens were captured for body measurements and blood samples, then released.

What the research team discovered was that the largest St Kilda and Shetland wrens were up to twice as heavy as their smallest mainland counterparts. While mainland wrens tip the scales at between seven and 10 grams, their St Kilda cousins are giants in comparison, ranging from 13 to 16 grams.

Though they looked outwardly similar, their genomes were found to be genetically distinct, demonstrating that there has been no recent breeding with mainland populations.

“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, then independently evolved to become island giants,” explained study author Michał Jezierski, a biogeographer at the University of Birmingham.

Furthermore, the wren populations on different islands have unique traits that evolved after isolation from their mainland ancestors. There’s variation in their plumage colours, body proportions, and songs, with more extreme features in the wrens of Shetland and St Kilda. Wrens from Fair Isle and the Outer Hebrides aren’t as far diverged from the British mainland wrens.

All islands obviously offer distinct ecological conditions, such as limitations on resources and lower predation and competition, absent from species present on continents. 

“Their genetic distinctiveness [on Shetland and St Kilda] is so high that it’s likely they’re on their way to becoming new species,” explained Jezierski.

Image content - Craig Nisbet