Tiny wrens on remote Scottish islands are growing much larger than their mainland relatives and developing distinct traits that may lead to new species.
Researchers at the University of Birmingham studied four isolated populations of wrens in Scotland, including those on Shetland, Fair Isle, the Outer Hebrides, and St Kilda. The birds on St Kilda now weigh between 13 and 16 grams, compared with 7 to 10 grams for mainland birds in Britain. The largest St Kilda wrens are more than twice the size of the smallest mainland specimens.