The Famous Oslo Grouse has proven quite popular with birders and it has been suggested that it is an even more exceptional record than I first thought.
When I saw the bird I did not consider that it was anything other than a Willow Grouse (lirype). Ptarmigan (fjellrype) looks pretty much identical in white winter plumage but the chance of this bird being that species did not warrant any thought, or so I thought. After all the habitat did not suit the high montane Ptarmigan and of the two species Ptarmigan is hardly ever seen seen away from its breeding sites whereas Willow Grouse is known to wander. It has however been pointed out that the bill of the Oslo bird is very small and in white winter plumage that is one of the few clues to separate these two species whose white plumage is identical. Male Ptarmigan has black lores (the area between the bill and eye) but female Ptarmigan and both sexes of Willow Grouse have white lores. The Oslo bird has white lores so this rules out male Ptarmigan but not female. Otherwise, Ptarmigan is a slightly smaller bird but size is notoriously difficult to judge on lone birds. The calls of the two species are slightly different but I have not heard it call and the one observer who has heard it heard a call unfamiliar to him for either species!
There is one other difference which is the colour of the claws. BWP describes the colour as blue-black for Ptarmigan and horn-brown for Willow Grouse. I have not found many photos on the web to confirm this although have not found anything to suggest this colour description is wrong. The colour of the Oslo birds claws are difficult to see due to a lot of feathering but this picture by Per Buertange shows them to be horn-brown. It has also been stated (this was a statement by one person which has not yet been supported by others) that exactly the excess feathering is a Willow Grouse character.
Bill size can be easily checked on photos on the web and it is clear that there is a huge amount of individual variation between the species with some Ptarmigan having in my eyes monster bills whereas some Willow Grouse (females?) have small bills that match the Oslo bird. I also took a picture on 12 Feb of a Willow Grouse in Hedmark that matches the Oslo bird. This bird was in a lowland, forest habitat with a flock of similar birds so is for me undoubtedly a Willow Grouse. Also BWP confirms the large individual variation with the following measurements being given:
Ptarmigan 9.0 - 10.6 8.2 - 9.8
Willow Grouse 10.0 - 12.8 9.8 - 12.3
So what do I think the bird is? Well I find the thought of it being a Ptarmigan so outrageous that I still believe it to be a small-billed female Willow Grouse which the claw colour supports. Here is a picture showing the bill of the Oslo bird (right) with a comparison to the bird I photographed in Hedmark. If the claw colour (and feathering of the claws) are not diagnostic then it is difficult to know how to 100% prove the ID of this bird. Trapping it to take measurements would be ethically wrong but it is probably possible to take DNA from its droppings.