Barn swallows evolved alongside buildings

New research analyses the genetics of barn swallows and how their evolution was affected by human settlements.

Adult swallow at the nest with chicks. © Kaphoto/Getty

One swallow might not make a summer, but 7,700 summers are apparently enough to make a swallow. New research traces the origin of the barn swallow to a key technological development in human civilisation – and, no, it’s not telephone wires. 

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“We know that barn swallow ecology is closely tied to humans, because they nest on human structures,” says biologist Chris Smith of the University of Colorado.

But, he says, our relationship with the birds is more “profound” than that. “Our results suggest that the subspecies did not exist before barns existed.”

Swallow resting on a fence. © Toni Poikeljärvi/Getty
Swallow resting on a fence. © Toni Poikeljärvi/Getty

The barn swallow that we know so well in the UK is just one of six subspecies that breed in various regions of the northern hemisphere. All but one of them migrate south for the winter.

It has long been wondered whether their evolution was prompted by the appearance of barns and other robust, man-made structures. 

Tentative studies carried out to date have suggested that barn swallows had expanded around the world long before humans started building permanent settlements. But a more detailed study, conducted by Smith and his colleagues, now suggests this isn’t the case.

The team analysed the genomes of the two most distantly related subspecies – one from North America and another from Egypt – to get a more precise picture of the species’ evolutionary history.

They found evidence of a genetic bottleneck – a sign of a very small population – about 7,700 years ago, which is around the same time as humans started building structures.

A group of small swallows in the nest. © Giuseppe Zanoni/Getty
A group of small swallows in the nest. © Giuseppe Zanoni/Getty

“Relatively few birds from the ancestral population may have first begun nesting on human structures,” says Smith.

“The stark ecological differences associated with this transition [might have] caused isolation from the ancestral population, in turn leading to genetic divergence.”

Among the questions now being tackled by Smith and his colleagues is whether the various subspecies of swallow are the result of birds following the development of human architecture as it spread around the world.

It is possible that being a migratory species may have been significant in helping facilitate its dispersal between continents. 

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Read the paper in Molecular Ecology.