Prisoners are helping house sparrows to live behind bars to try to slow their decline.

A colony of Passer domesticus has found sanctuary with three endangered lowland gorillas at London Zoo. Ten nesting boxes are being built for the birds at Spring Hill Prison near Aylesbury in time for the spring.

A spokeswoman for the Prison Service said: “Many prisoners get satisfaction from community projects that involve skill and commitment. Prisoners making the boxes are able to learn a series of skills including maths, carpentry, health and safety. They have also conducted environmental impact assessments.”

Jonathan Baillie, head of field conservation at the Zoological Society of London, said: “Sparrows used to be a common sight in London, earning their reputation as the ‘cockney sparra’. Unfortunately this is no longer the case and these delightful birds need help to prevent their British population declining further. The new boxes in Gorilla Kingdom will encourage the colony to breed and remain here.”

Gorillas and sparrows go back a long way at the zoo. Guy the Gorilla, who arrived in 1947 and lived there for more than 30 years, was known for his habit of picking up sparrows, inspecting them and then putting them back down gently.

In the 1950s the number of house sparrows in Britain was estimated at 9.5 million, rising to 12 million by the early 1970s. In the past 25 years the number has crashed by 62 per cent. Researchers suspect that the fall may be linked to a shortage of insects in summer.

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