Migratory swifts loyally return every year to their nests in buildings, according to a study, underlining the importance of providing the endangered birds with hollow nesting bricks if traditional nest sites are lost to renovations.

The swift, which is on the red list of conservation concern, is one of Britain’s most threatened species, having declined in number by 70% since 1995 because of the loss of nesting sites, often when old buildings are re-roofed or given better insulation. While Scotland this year made the installation of swift bricks – a simple hollow brick – a legal requirement in new buildings, the government in England has repeatedly refused to oblige builders to include a £35 swift brick in every new home.

Scientists for the RSPB studied 190 different swifts from 243 nests over 15 years in a Dartmoor village and found that 94% of the birds reused the same nesting site as the previous year.

The 15-year study found that swifts, whose screaming parties over towns and cities are a much-loved sound of summer, have greater loyalty to their nest site than a previous partner, with only six out of 10 (59%) birds pairing with the same partner. Sometimes, as nest camera footage vividly reveals, there are fierce debates over the use of a nesting box.

Researchers collected data from swifts breeding in nest boxes in Drewsteignton, Devon. Breeding swifts were identified by uniquely numbered leg rings that enabled scientists to track which birds bred together and which nest box each swift used every year. Data was collected from 243 nests during this period.

Malcolm Burgess, the principal conservation scientist for the RSPB, said: “We’ve anecdotally thought for a long time that swifts are loyal, returning to the same nesting sites and partners each spring. But for the first time, we’ve documented just how strongly faithful they are to their nest sites, which highlights just how important it is to protect their nesting sites in our neighbourhoods.

“The decline in swift numbers is a great cause for concern and without increasing the availability of nest sites, and replacing those that are being lost, we will see further declines of this extraordinary bird in our towns and cities.”

This spring, holes that had been blocked in a railway viaduct in Derbyshire were opened up again by Network Rail for the swifts after a local outcry. But swifts returning to their nest sites in Dorking, Surrey, found their ancestral home was demolished by contractors during the nesting season.

The naturalist and writer Hannah Bourne-Taylor has conducted a four-year campaign to make one swift brick obligatory in every new home, winning in Scotland but finding the Labour government reversing its previous support for the measure.

The study has been published just before UK Swift Awareness Week, in which more than 150 local swift groups from Aberdeen to Devon put on events to help people understand how they can help the birds.

Nick Brown, the coordinator of Swift Awareness Week, said this year swiftlets may be found on the ground if the current heatwave causes them to overheat in their nests. “What will be critical to their breeding success this year is the extent to which this really hot spell causes chicks to die in the nests. That’s the next little crisis for them,” he said.

Local swift groups have volunteers who will care for the swiftlets if they are rescued – once a swift is on the ground it cannot usually get airborne again without help.

According to Brown, while the swift has not had much help from government, it is receiving ever more support from community groups and local people. “The numbers of swift groups and events keeps growing. The interest and enthusiasm and care for swifts is growing exponentially,” he said. “Every year more people get involved and take action.”