Wrens on isolated Scottish islands have evolved to be significantly larger than mainland wrens due to a lack of predators, a phenomenon known as island gigantism, and scientists have found they are genetically distinct.

Wrens on Shetland, St Kilda, and other Scottish islands have developed gigantism due to lack of predators, with St Kilda wrens weighing 13-16g (twice the size of mainland birds at 7-10g) and Shetland wrens 2.9g heavier, leading Dr Michał Jezierski of the University of Birmingham and co-author Will Smith of the University of Nottingham to find that these subspecies are genetically distinct and likely evolving into new species, as published in the Evolutionary Journal of the Linnean Society.