Image caption A chance encounter put this finch back on biologists' map

It has been missing for 80 years but Sillem's Mountain Finch has now been rediscovered on the Tibetan plateau by a trekker who was too ill to leave camp.

The mountain finch has been an enigma ever since its discovery in 1929, not least because it wasn't identified until 1992.

Two specimens of the sparrow-sized grey and white bird with a russet head were collected by Dutch ornithologist Jerome Alexander Sillem on an expedition to the Karakoram mountain range in 1929.

Nowadays this is the disputed border region of China, India and Pakistan and a no-go area for birders.

The specimens were labelled as a race of Brandt's Mountain Finch (Leucosticte brandti) and consigned to a drawer in the Amsterdam Zoological Museum.

And there they remained until 1992 when a modern-day Dutch ornithologist, Kees Roselaar, opened the drawer and realised the two specimens were a distinctive species in their own right. And he named the new species Leucosticte sillemi - after the original finder.

Fortunate misfortune

But then the trail went cold - until June this year when French nature photographer Yann Muzika was trekking in the Yenigou valley of Qinghai province in China. However, he contracted food poisoning on the eve of departure and was soon confined to camp.

Yann takes up the story: "After the second day, I decided to take a day break and explore the surroundings as much as my condition would allow.

"It was a trek, not a birdwatching trip, but I was nevertheless carrying a camera and a 400mm lens, just in case.

"I came across a flock of Tibetan Rosefinches (Carpodacus roborowskii) and with them there was a single bird that I did not know, resembling a Brandt's Mountain Finch but with a rufous head instead of dark brown. I took one picture before the bird flew away.

Fantastic! At last the proof that sillemi still exists Kees Roselaar

"On my return home, I just downloaded the pictures and left them for a few weeks. I still couldn't identify the finch but in the Birds of China field guide there was a brief description of Sillem's Mountain Finch that seemed to match pretty well... but then we were talking of a bird that had not been seen since 1929.

"As I was reaching the limits of my expertise on birds, I sent the picture and others taken during the trek to Krys Kazmierczak who manages the Oriental Bird Images database for the Oriental Bird Club."

He immediately realised the significance of the "mystery bird" photo that had been emailed to him.

He told BBC News: "When I saw the excellent photo of the mystery bird my immediate thought was Sillem's Mountain Finch! However, being of a cautious disposition I did quite a bit of checking and consultation with others.

"Now we are pretty sure that it is Sillem's Mountain Finch, especially since it has been endorsed by Kees Roselaar, who simply said: 'Fantastic! At last the proof that sillemi still exists'."

The June 2012 bird was found 1,500km to the east of the original sighting in 1929 and the Oriental Bird Club is now urging birdwatchers to search for the bird above 5,000m over a vast swathe of high altitude Pakistan, China and Tibet.