By Ella Davies
Earth News reporter
The rare female (left) lacks the common white feathers of her mate (right)
A giant bird living in Germany has settled debate over the existence of a huge, dark species of sea eagle.
For over a century, experts questioned whether two Steller's sea eagle species exist: one with white feathers and a darker one.
But a dark, captive Steller's sea eagle in a Berlin zoo, the only living bird of its kind, has solved the mystery.
Born to white feathered parents, the dark Steller's sea eagle confirms they are two variants of the same species.
The Steller's sea eagle (Haliaeetus pelagicus) is the heaviest of all eagles, and it usually has brown and white plumage, sporting white feathers along the wings, legs and tail.
However, a different, dark form of the Steller's sea eagle was first described as a separate species (H. niger) in 1887. It had brown feathers all over except for a white tail.
Ornithologists suspected this darker Steller's sea eagle bred in Korea.
But few authenticated records exist, leading many to presume its extinction.
The last known dark Steller's sea eagle sighted in the wild was in 1968, and no dark specimen has been held in captivity since the start of the 20th Century.
Since these sightings, debate has continued as to whether the dark Steller's sea eagle is a separate species, subspecies or just a colour variant of the usual bird.
Most interested experts believe the latter, but no proof existed.
That was until the appearance of a dark Steller's sea eagle in Tierpark zoo, Berlin.
It wasn't until the Steller's sea eagle moulted into its adult plumage that the Tierpark's Curator of Birds, Dr Martin Kaiser, realised what a rarity it was.
The more common form of Steller's sea eagle
"It's really a surprise if you suddenly have a bird which was considered extinct and not observed for about half a century neither in the wild nor in captivity," says Dr Kaiser.
The eagle moulted into its adult plumage this year with only a white tail, making it the only known living bird of its kind.
The rare female was the product of artificial insemination at a falconry in Bavaria, Germany.
It arrived at Berlin's Tierpark in 2001 after being foster-reared by two American bald eagles at Nuremburg Zoo.
Crucially, its actual parents were wild birds, caught in Russia in the 1980s and both displayed the familiar white shoulder, leg and rump feathers.
The female is the only known living dark Steller's sea eagle
"Both the parents of the dark female in Tierpark Berlin show the normal coloured plumage with white shoulders and rump," says Dr Kaiser.
"This is the evidence that it is a colour phase only... [For the female to] be a subspecies the parents must be also dark coloured."
As the offspring of two white-marked birds, the Tierpark's female provides the first evidence that the dark plumage is not species specific, and that the dark eagles do not exist as a species in their own right.
The fact that the parents came from Russia also proves that dark forms of the eagle are not restricted to Korea.
Dr Kaiser's findings are published in the Journal of Ornithology.
The Steller's sea eagle is one of the world's largest eagles and certainly the heaviest weighing up to 9kg.
Competition for the top title comes from the Philippine eagle (Pithecophaga jefferyi) of South-East Asia; the martial eagle (Polemaetus bellicosus) of Africa and the harpy eagle (Harpia harpyja), of North and South America.
There are approximately 5,000 Steller's sea eagles in the wild, predominantly found in north-eastern Asia, breeding around the Kamchatka peninsula in Russia.
The giant birds make their nests along Russia's Pacific coast close to their preferred salmon feeding grounds.
Their distinctive large yellow bills are perfect for ripping flesh and stealing food from other eagles.
The IUCN list of endangered animals describes the sea eagles as vulnerable with a declining population under threat from habitat destruction and over-fishing.
Each winter many migrate to the Japanese islands of Hokkaido where they are known as O-Washi, and protected as a species of national importance.