Scottish Island Wrens and the Evolution of New Species

Imagine a wren: brown, restless and so light it barely seems to weigh anything. It is the sort of bird that disappears into a thicket before you have even noticed it. In Britain, one weighs roughly the same as two sugar cubes placed together.

That makes the news from a few storm-battered Scottish islands particularly surprising: some of their wrens weigh more than twice as much. More unexpectedly still, scientists now believe the birds may be on their way to becoming something entirely new.

Scottish island wrens become giants

The findings come from a team led by Dr Michał Jezierski at the University of Birmingham (UoB). The researchers studied four isolated populations of the Eurasian wren (Troglodytes troglodytes) living around Scotland’s northern and western fringes.

The birds inhabit Shetland, Fair Isle, the Outer Hebrides and the tiny, remote outpost of St Kilda. A mainland wren weighs approximately seven to ten grams, whereas those on St Kilda weigh 13 to 16 grams.

The biggest island birds are more than twice as heavy as the smallest mainland wrens, placing them among the heaviest examples of their kind anywhere in the world.

This marked increase in size is known as island gigantism. It occurs when an animal confined to an island grows much larger than its mainland relatives.

Over time, the phenomenon has produced some of nature’s most unusual inhabitants, including the giant tortoises of the Galápagos and the extinct dodo. It is genuinely surprising to find a small brown songbird joining their ranks.

What the genes revealed

To establish what was taking place, the researchers measured the birds’ bodies, recorded their songs and read the complete genetic code of wrens from each island and the mainland.

Together, those approaches revealed not only differences in appearance but also how far the populations had diverged beneath the surface.

Each of the four Scottish island groups proved genetically distinct from mainland wrens.

The greatest differences were found in the Shetland and St Kilda birds, whose bodies, songs and DNA set them apart. There was little evidence that these island wrens breed with mainland birds at all.

Isolation is central to the process. When a population remains cut off for long enough, with few predators and little competition, it begins to follow its own evolutionary path.

“Their genetic distinctiveness is so high,” said Jezierski, “that they are likely on their way to becoming new species.”

Two roads, one shape

One finding took the researchers by surprise. Wrens from Shetland and St Kilda look strikingly similar: both are unusually large and greyer than their mainland relatives.

It would be reasonable to suppose that they became bigger in the same way. Yet that is not what happened.

Most of the genetic changes responsible for their greater size differ between the two populations. Each island reached a similar outcome by a separate route, an example of parallel evolution.

In this process, comparable pressures lead to similar results through different means.

As Jezierski explained, colonising birds probably arrived independently in each island archipelago. They then grew into giants separately, while their songs diverged from those of mainland wrens.

Slim genetic overlap

This addresses a question that researchers in the field had not previously settled.

Scientists have long argued over whether similar body shapes necessarily result from matching DNA changes. In these wrens, the evidence points towards a negative answer.

Only a small fraction of the most distinctive genetic regions overlapped between the two giant island populations: roughly four per cent. For the rest, the wrens on each island followed their own genetic path.

When islands disagree

Not every Scottish island produced giant wrens, and that proved just as revealing.

In both size and genetics, the wrens of Fair Isle and the Outer Hebrides remained much closer to their mainland relatives.

Fair Isle was particularly surprising. Although it lies between Shetland and the mainland, its wrens showed no meaningful increase in size.

The island sits on a busy migration route, so birds passing through from elsewhere may continue to mix with the local population. That could blur distinctions that become sharper on other islands.

Evolution on islands does not follow a fixed timetable: even islands a short distance apart can send their wrens along entirely different paths.

Geography provides the setting, but it does not determine every outcome.

Island evolution in action

Islands are home to a remarkable share of life on Earth, containing somewhere between a fifth and a third of all known species. They have long been regarded as natural laboratories for studying evolution.

These wrens provide a clearer view of the processes at work underneath.

A co-author from the University of Nottingham (UoN) described the birds as a powerful case study in how island life develops around the world.

Before this research, nobody had demonstrated as clearly that birds can develop the same dramatic body type while their genomes take different paths.

A recent study of island songbirds suggested that the process was complex; the wrens give that complexity a concrete example.

Why the wrens matter now

The finding changes how biologists think about predicting evolution, because the same result can conceal very different genetic causes.

The researchers say it remains unclear whether genetic drift or selection drove the changes.

For now, the main shift is in where attention is directed. The four populations provide a living model for investigating how new species arise.

Conservationists may also have to consider whether birds this distinctive warrant protection as something more than a regional variant.

The wrens have not yet become new species. But on islands small enough to photograph from a single hillside, that threshold is drawing nearer.