A snippet of film shot by an amateur ornithologist in 1956 is providing researchers and bird enthusiasts with a rare opportunity to observe the behaviour of the imperial woodpecker, a giant resident of Mexico's Sierra Madre now presumed extinct.

The Cornell Lab of Ornithology released the colour film to the public on its website Wednesday. The 85-second film shows the bird foraging for insects on several giant pines and flying from tree to tree. It is the only known photographic record of the 60 centimetre-tall bird in the wild.

"It's the last confirmed sighting of an extinct species. It's a point in geography and time where you can document the species existed," said Joel Cracraft, curator of birds at the American Museum of Natural History in Manhattan.

The film was shot by the late William Rhein, a Pennsylvania dentist who made three expeditions to find the bird in Mexico's Sierra Madre Occidental before he finally captured one on film. Cornell woodpecker specialist Martjan Lammertink learned of the film through an old letter in Cornell's archives.

"It is stunning to look back through time with this film and see the magnificent imperial woodpecker moving through its old-growth forest environment. And it is heartbreaking to know that both the bird and the forest are gone," said Lammertink, lead author of a paper about the film published in this month's edition of The Auk, the journal of the American Ornithologist's Union.An illustration provided by artist John Schmitt shows the extinct imperial woodpecker, which once lived in the old growth pine forests of Mexico's Sierra Madre Occidental. (John Schmitt/Associated Press)

The imperial woodpecker was the closest relative of the ivory-billed woodpecker. A 2005 study by the Cornell Lab reported the rediscovery of an ivory-billed woodpecker in Arkansas, but subsequent surveys didn't find evidence of a surviving population.

The imperial woodpecker is mostly black with wings half-white, a large white beak, and a pointed crest that is red on the male and black on the female. The film is somewhat blurry and shaky because Rhein was riding on a mule, but the scientists analyzed it and verified the sighting.

"It's amazing that the bird does so many different things in 85 seconds," said Tim Gallagher, editor of the lab's "Living Bird" magazine and author of a book about the imperial woodpecker to be published next year. "It's a gold mine of information about this bird."

Lammertink tracked down Rhein more than 10 years ago in Mechanicsburg, Pennsylvania, to see the film. Rhein's nephew donated the film to Cornell's ornithological library in 2006, after Rhein's death.

Gallagher and Lammertink made a research trip last year to the location where the film was shot, but found no trace of the bird. Instead, they found most of the massive old-growth pines cut down and an unwelcoming environment.

"It's a very dangerous place, in the heart of opium- and marijuana-growing country. There were a lot of people with AK-47s," Gallagher said.

Scientists believe the bird became extinct because it was hunted for food and its habitat was destroyed.