Some swifts returning to Britain to breed will be unable to access their ancestral nesting holes after they were blocked in a £7.5m refurbishment of a Derbyshire railway viaduct, campaigners say.

Nature lovers had appealed to Network Rail to unblock three holes which were among at least nine swift nesting sites on the twin viaducts at Chapel Milton, on the edge of the Peak District.

Campaigners said Network Rail had been given extensive details of the location of the nest holes, between tiny gaps in the stonework, but three were filled with mortar in February.

“It’s utterly heartbreaking,” said Deb Pitman, a swift campaigner. “Seeing footage of what happens when swifts return to a blocked nest is deeply upsetting. They repeatedly try to access their entry point, sometimes with fatal consequences.

“It is thought established breeding pairs do not successfully relocate to a new nest site. In effect, when a nest site is lost, they stop breeding,”

Network Rail began work on the refurbishment of the 160-year-old viaducts last summer while swifts were nesting. An ecological report, commissioned by Network Rail and not required by law, found no evidence of nesting swifts.

This claim was challenged by swift watchers, who assembled 38 pieces of evidence they said showed at least nine nest sites on the viaducts. Network Rail later accepted there were swift holes and said it had worked around the nesting birds.

British Transport Police’s wildlife crime team was investigating whether nesting birds may have been disturbed, after the refurbishment work was reported to the police by campaigners last summer.

A Network Rail spokesperson said: “During essential refurbishment work on the viaduct at Chapel Milton last year, our teams identified potential swift nests. We consulted ecologists and created exclusion zones to ensure adequate protection for the nesting swifts. The essential repairs to the masonry were then completed after the bird nesting season.

“We welcome the opportunity to work with local groups to make the viaduct a more welcoming habitat for swifts, including installing additional nest boxes alongside existing nest sites.”

Campaigners say preserving the tiny swift holes would have no impact on the structural integrity of the viaduct.

Jason Adshead, from the Chapel-en-le-Frith Biodiversity Group, said: “We spent a great deal of time monitoring the work at the viaduct last year and we have evidence showing where the nest sites are. It’s information we shared with Network Rail.

“We sincerely hope they will arrange for the nest holes to be reopened before the birds return in numbers. Swifts are the sight and sound of summer and we are incredibly lucky to have them here.”

Bird lovers planned to hold a vigil beneath the viaduct on Saturday in the hope that the holes would be unblocked before the swifts returned around the end of April to breed.

Britain’s swift population has declined by 66% between 1995 and 2022, largely due to the widespread loss of nesting holes in older roofs and buildings. A national campaign seeking to ensure all new homes are built with at least one hollow “swift brick” has been successful in Scotland but was rejected by the Labour government in Westminster.

Pitman said: “Swifts have been successfully breeding since around the time of the dinosaurs and yet we can’t afford them a tiny space. I refuse to accept it.”