It is a feat of endurance almost as impressive as the migratory flight of a swallow.

Eastern Daily Press: A Cuckoo.A Cuckoo. (Image: BTO)

A bird enthusiast from Norfolk has spent almost 4,000 hours on a project to digitise a library of tens of thousands of bird photographs for the Thetford-based British Trust for Ornithology (BTO).

Doug Mullings started volunteering at the organisation in 2003 and, since then, has scanned in and labelled a quarter of a million slides, helping to preserve the images for the future.

The vast majority of the pictures are of birds, although they include other images from the organisation's archives, which stretch back to its foundation in the 1930s.

Officials at the BTO have calculated that if the images were stacked on top of each other, they would create the second tallest man-made structure on the planet – at 750 metres – behind only the Burj Khalifa tower, in Dubai (830 metres).

Eastern Daily Press: This 1949 photo shows an ornithologist photographing a nest box.This 1949 photo shows an ornithologist photographing a nest box. (Image: BTO)

The efforts of Mr Mullings, from St John's Way, in Thetford, has now been recognised by the organisation, which has awarded him with a medal for his service.

He was presented with the decoration by wildlife broadcaster Chris Packham.

Mr Mullings, 65, said: 'After a serious car accident left me unable to work, finding any work became almost impossible and I lost a sense of worth. Spending time helping the BTO brought that back to me.

'Checking all of the bird images that go into the BTO library gives me great pleasure, so I find it overwhelming to be recognised in this way, and I am extremely pleased to receive the BTO medal.'

Eastern Daily Press: Great Spotted Woodpecker.Great Spotted Woodpecker. (Image: BTO)

Paul Stancliffe, from the BTO, said: 'Without the huge amount of time Doug has given to the trust, our image library would be in a much poorer state. He is a very worthy winner of the BTO's Jubilee Medal.'