Science Matters – Uncovering the history of the British Barn Owl

Bob Shirley with the barn owl shortly after rescuing it from the sea. Photograph: Bob Shirley. NO F13 barn owl at Arisaig 02

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The British barn owl (Tyto alba) is a fairly common nocturnal bird in rural areas of the UK, easily recognisable in the dark by its eerily pale plumage.

But why is this night-flying hunter predominately white in colour when its close neighbours in Europe are mainly dark rufous?

This question has puzzled ecologists, but now the DNA sequencing of 147 genomes from six European barn owl populations has provided the answer*.

At the beginning of the last ice age when an ice sheet crept down over northern Europe, barn owls, along with many other animal species, including humans, migrated south.

The owls found refuge mainly in the Italian, Iberian and the Balkan peninsulas, but then, when temperatures began to rise some 18,000 years ago, these species slowly returned to recolonise the northern regions.

Most animals arrived in the UK from northwest Europe via Doggerland, a land-bridge joining UK to today’s Belgium and Denmark when sea levels were low during the ice age, but which submerged around 8,000 years ago.

However, we now know that British barn owls took an alternative post-glacial colonisation route to reach the UK.

The genome analysis identified Portuguese barn owls as the most genetically diverse, including white and rufous individuals, and as the founder population that eventually recolonised Europe after the ice age.

Barn owls returned to north-western Europe an estimated 10,000 years ago via the land route across the Pyrenees mountains.

Then, around 6,000 to 7,000 years ago, a genetically separate group of white barn owls reached the UK from Portugal by a hitherto unknown westerly pathway, utilising habitable islands in the Bay of Biscay that were later submerged by rising sea levels as the ice cap melted.

Sea crossings seem rare among barn owls, as evidenced by the distinct populations in the UK and Europe.

And although owls from Britain later reached Ireland, only a few made the journey and it is suggested that they island hopped from western Scotland to become another distinct population.

*Machado A-P et al. Molecular Ecology. 2021;001-16.