The owlets were born to a breeding pair of barn owls which have made Tarmac's Harden Quarry at Biddlestone, near Netherton, their home.

The owls came to the quarry last year after a nest box was erected and raised two young. Another two were born this year.

Martin Taylor, Harden's quarry manager, said: "We erected the nest box in 2007 in the hope that we would attract a breeding pair of Barn Owls.

"We never expected them to take to it so soon and breed so successfully, it obviously suits their needs perfectly.

"In the first year they raised two young and this year they successfully raised a further two owlets which have both fledged."

Steve Lowe, head of conservation at Northumberland Wildlife Trust, said: "Barn Owls have suffered a decline over the past 50 years as a result of the degradation of once prey-rich habitats due to intensive agricultural practices in many parts of the UK.

"It's fantastic to see a company like Tarmac encouraging their breeding habits and preserving the wildlife in and around their quarries."

Tarmac is the first quarrying company to have Biodiversity Action Plans in place at all its 120 active quarries across the UK, with Northumberland being the first county to complete the programme in last year.

The plan sets down how to protect and enhance the biodiversity within and around the quarries.