Great Northern Diver

Today I joined the twitch of a Great Northern Diver (Gavia immer) at Neue Donau, Vienna. The bird was reported as an adult yesterday and was still present at noon. It showed very well at 70-100m most of the time, though it sometimes seemed a little restless between the 15+ birders watching from both riversides.
Although sources on ageing of the species (extending beyond the differentiation of juveniles and adults) are limited and captions of photos on the net often seem contradictory, I think that the bird in question could be a subadult, in 3rd or 4th calendar-year. The eye seems too dark for a full adult, which should show a brighter reddish-brown color.
Since 1980, there have been 36 records of Great Northern Diver from Austria ( of which one was at the same location in November 1998 (Ranner, A. 2002: Nachweise seltener und bemerkenswerter Vogelarten in Österreich 1996- 1998. 3. Bericht der Avifaunistischen Kommission von BirdLife Österreich. Egretta 45: 1- 37.)